Vulkan Instanced Rendering Weird Depth Buffer Behaviour - vulkan

Question

I am rendering a single mesh test object that contains 8000 cubes, 1 monkey, and 1 plane. I checked normals and winding orders. all seems correct for mesh.
if I render a single instance, the depth buffer works correctly. if I render using instanced when instant number increase depth buffer doesn't work correctly.
//Depth Attachment
//Create Depth Image
depthImage = new Image(
physicalDevice_,
logicalDevice_,
VK_IMAGE_TYPE_2D,
depthFormat,
VK_IMAGE_TILING_OPTIMAL,
swapChainExtent.width,
swapChainExtent.height,
1,
sampleCount_, //do we really need to sample depth????
VK_IMAGE_LAYOUT_UNDEFINED,
VK_IMAGE_USAGE_DEPTH_STENCIL_ATTACHMENT_BIT,
VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT);
depthImageView = new ImageView(
logicalDevice_,
VK_IMAGE_VIEW_TYPE_2D,
depthImage->handle,
depthFormat,
1,
VK_IMAGE_ASPECT_DEPTH_BIT);
//Render pass Depth Attachment
VkAttachmentDescription depthAttachment{};
depthAttachment.format = depthFormat;
depthAttachment.samples = VK_SAMPLE_COUNT_1_BIT;
depthAttachment.loadOp = VK_ATTACHMENT_LOAD_OP_CLEAR;
depthAttachment.storeOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
depthAttachment.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
depthAttachment.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
depthAttachment.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
depthAttachment.finalLayout = VK_IMAGE_LAYOUT_DEPTH_STENCIL_ATTACHMENT_OPTIMAL;
//Graphics Pipeline
VkPipelineDepthStencilStateCreateInfo depthStencil{};
depthStencil.sType = VK_STRUCTURE_TYPE_PIPELINE_DEPTH_STENCIL_STATE_CREATE_INFO;
depthStencil.depthTestEnable = VK_TRUE;
depthStencil.depthWriteEnable = VK_TRUE;
depthStencil.depthCompareOp = VK_COMPARE_OP_LESS;
depthStencil.depthBoundsTestEnable = VK_FALSE;
depthStencil.minDepthBounds = 0.0f; // Optional
depthStencil.maxDepthBounds = 1.0f; // Optional
depthStencil.stencilTestEnable = VK_FALSE;
depthStencil.front = {}; // Optional
depthStencil.back = {}; // Optional
Below the screen is the view from the 100th instance of that mesh.
The higher the instance number, the more the depth buffer deteriorates.
The result is still the same if I use a different model.
What am I missing?
Result:
Depth Buffer:

Answer 1

For those who may struggle with similar issue. I found a solution.
Actually, this issue is not related to instancing directly but it's a bit confusing issue becomes visible as the instance index increases.
the cause is projection matrix z-near value was so small.
...
fieldOfView = 65.0f;
znear = 0.001f; // ! this much small value cause this problem
zfar = 5000.0f;
...
changed z-near to an acceptable value. Problem solved.
...
fieldOfView = 65.0f;
znear = 0.1f;
zfar = 5000.0f;
...