Inexplicably cleared msbuild properties in TeamCity build - msbuild

Question

I'm trying to create a "tools NuGet package" that provides a tool and setting that is unpacked during build and used by a later TeamCity build step.
The NuGet package contains the following content in its build\MyPackageId.props file:
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup>
<MyTool1>$(MSBuildThisFileDirectory)..\tools\MyTool.exe</MyTool1>
</PropertyGroup>
<Target Name="ReportMyToolToTeamCity" BeforeTargets="PrepareToRun">
<PropertyGroup>
<MyTool2>$(MSBuildThisFileDirectory)..\tools\MyTool.exe</MyTool2>
</PropertyGroup>
<Message Text="MyTool1 = $(MyTool1)" />
<Message Text="MyTool2 = $(MyTool2)" />
</Target>
</Project>
(The messages will eventually set a TeamCity property, but this is sufficient to demonstrate the issue.)
Because it's a props file, after installing the NuGet package into a C# project it has added an import as the very first thing, above the import of Microsoft.Common.props. I want a props file rather than a targets file so that the property values are also available to other project settings and targets files.
When I compile this inside Visual Studio 2015, I see both MyTool1 and MyTool2 paths set to the same (correct) path as expected.
When I compile this from TeamCity (2017.2.2, using the Visual Studio (sln) runner), according to the output the MyTool1 property is empty and only MyTool2 shows the correct value.
Why?