EventRule matched by default EventBridge EventBus ignored by custom EventBus - amazon-cloudwatch

Question

I have a Cloudformation template which implements a CI/CD process for Lambda functions (at bottom)
Essentially it -
watches a Github repo
pulls source code on new git tag creation
starts a CodeBuild process which runs unit tests, zips source code (on test success) and pushes archive to S3
enables CodeBuild notifications
implements a CloudWatch EventRule to pattern match raw CodeBuild notifications, format and push them to SNS
binds a Lambda function to SNS, which pushes the notifications to Slack via a webhook
This works fine with the default EventBridge EventBus, but the pattern matching seems to fail if I switch to a custom EventBus.
(see resources EventBus and EventRule in the stack; it's currently set up to use the custom EventBus, and fails / ignores new git tags; if you comment out the reference to property EventBusName in EventBus, it defaults to using the default EventBus and works)
Why would a custom EventBus behave differently to the default EventBus in this situation ?
---
AWSTemplateFormatVersion: '2010-09-09'
Parameters:
AppName:
Type: String
RepoOwner:
Type: String
RepoName:
Type: String
RepoBranch:
Type: String
Default: master
RepoAuth:
Type: String
Default: master
WebhookUrl:
Type: String
WebhookLambda:
Type: String
CodeBuildBuildSpec:
Type: String
CodeBuildType:
Type: String
Default: LINUX_CONTAINER
CodeBuildComputeType:
Type: String
Default: BUILD_GENERAL1_SMALL
CodeBuildImage:
Type: String
Default: aws/codebuild/standard:4.0
LambdaHandler:
Type: String
Default: "index.handler"
LambdaMemory:
Type: Number
Default: 128
LambdaTimeout:
Type: Number
Default: 30
LambdaRuntime:
Type: String
Default: python3.8
Resources:
ArtifactsBucket:
Type: AWS::S3::Bucket
Properties:
BucketName:
Fn::Sub:
- ${app_name}-lambda-artifacts
- app_name:
Ref: AppName
CodeBuildProject:
Properties:
Environment:
ComputeType:
Ref: CodeBuildComputeType
Image:
Ref: CodeBuildImage
Type:
Ref: CodeBuildType
Name:
Fn::Sub:
- ${app_name}-lambda-ci
- app_name:
Ref: AppName
ServiceRole:
Fn::GetAtt:
- CodeBuildRole
- Arn
Source:
Auth:
Resource:
Ref: RepoAuth
Type: OAUTH
Location:
Fn::Sub:
- "https://github.com/${repo_owner}/${repo_name}.git"
- repo_owner:
Ref: RepoOwner
repo_name:
Ref: RepoName
Type: GITHUB
BuildSpec:
Fn::Sub:
- "${build_spec}"
- build_spec:
Ref: CodeBuildBuildSpec
Artifacts:
Type: NO_ARTIFACTS
SourceVersion:
Ref: RepoBranch
Triggers:
Webhook: true
FilterGroups:
- - Type: EVENT
Pattern: PUSH
ExcludeMatchedPattern: false
- Type: HEAD_REF
Pattern: "refs/tags/.*"
ExcludeMatchedPattern: false
Type: AWS::CodeBuild::Project
CodeBuildRole:
Properties:
AssumeRolePolicyDocument:
Statement:
- Action: sts:AssumeRole
Effect: Allow
Principal:
Service: codebuild.amazonaws.com
Version: '2012-10-17'
Policies:
- PolicyDocument:
Statement:
- Action:
- codebuild:*
- events:*
- s3:PutObject
- logs:CreateLogGroup
- logs:CreateLogStream
- logs:PutLogEvents
Effect: Allow
Resource: '*'
Version: '2012-10-17'
PolicyName: code-build-role-policy
Type: AWS::IAM::Role
WebhookFunction:
Properties:
FunctionName:
Fn::Sub:
- ${app_name}-lambda-webhook
- app_name:
Ref: AppName
Code:
ZipFile:
Ref: WebhookLambda
Environment:
Variables:
WEBHOOK_URL:
Ref: WebhookUrl
Handler:
Ref: LambdaHandler
MemorySize:
Ref: LambdaMemory
Role:
Fn::GetAtt:
- WebhookFunctionRole
- Arn
Runtime:
Ref: LambdaRuntime
Timeout:
Ref: LambdaTimeout
Type: AWS::Lambda::Function
WebhookFunctionRole:
Properties:
AssumeRolePolicyDocument:
Statement:
- Action: sts:AssumeRole
Effect: Allow
Principal:
Service: lambda.amazonaws.com
Version: '2012-10-17'
Policies:
- PolicyDocument:
Statement:
- Action:
- logs:CreateLogGroup
- logs:CreateLogStream
- logs:PutLogEvents
Effect: Allow
Resource: '*'
Version: '2012-10-17'
PolicyName: webhook-role-policy
Type: AWS::IAM::Role
WebhookFunctionPermission:
Properties:
Action: "lambda:InvokeFunction"
FunctionName:
Ref: WebhookFunction
Principal: "sns.amazonaws.com"
SourceArn:
Ref: WebhookTopic
Type: AWS::Lambda::Permission
WebhookTopic:
Properties:
Subscription:
- Protocol: lambda
Endpoint:
Fn::GetAtt:
- WebhookFunction
- Arn
Type: AWS::SNS::Topic
WebhookTopicPolicy:
Properties:
PolicyDocument:
Version: "2012-10-17"
Statement:
- Effect: Allow
Principal:
Service: "events.amazonaws.com"
Action:
- "sns:Publish"
Resource:
Ref: WebhookTopic
Topics:
- Ref: WebhookTopic
Type: AWS::SNS::TopicPolicy
EventBus:
Type: AWS::Events::EventBus
Properties:
Name:
Fn::Sub:
- ${app_name}-bus
- app_name:
Ref: AppName
EventRule:
Type: AWS::Events::Rule
Properties:
EventBusName: # CURRENTLY USING CUSTOM EVENT BUS (PATTERN MATCHING FAILS); REMOVE THIS PROPERTY TO SWITCH TO DEFAULT EVENT BUS (PATTERN MATCHING WORKS)
Ref: EventBus
EventPattern:
source:
- "aws.codebuild"
detail-type:
- "CodeBuild Build Phase Change"
detail:
completed-phase:
- SUBMITTED
- PROVISIONING
- DOWNLOAD_SOURCE
- INSTALL
- PRE_BUILD
- BUILD
- POST_BUILD
- UPLOAD_ARTIFACTS
- FINALIZING
completed-phase-status:
- TIMED_OUT
- STOPPED
- FAILED
- SUCCEEDED
- FAULT
- CLIENT_ERROR
project-name:
- Ref: CodeBuildProject
State: ENABLED
Targets:
- Arn:
Ref: WebhookTopic
Id:
Fn::Sub:
- "${project_name}-codebuild-notifications"
- project_name:
Ref: CodeBuildProject
InputTransformer:
InputPathsMap:
build-id: "$.detail.build-id"
project-name: "$.detail.project-name"
completed-phase: "$.detail.completed-phase"
completed-phase-status: "$.detail.completed-phase-status"
InputTemplate: |
"{'build-id': '<build-id>', 'project-name': '<project-name>', 'completed-phase': '<completed-phase>', 'completed-phase-status': '<completed-phase-status>'}"

Answer 1

"Custom event buses serve a use case of receiving events from your custom applications and services. Unfortunately, it is not possible for AWS services to push events to a custom event bus." (AWS Support)