There is in fact a feedback loop problem

		GPU_framebuffer_ensure_config(&fbl->composite_fb, {
			GPU_ATTACHMENT_TEXTURE(dtxl->depth),
			GPU_ATTACHMENT_TEXTURE(e_data.composite_buffer_tx),
		});

static void workbench_composite_uniforms(WORKBENCH_PrivateData *wpd, DRWShadingGroup *grp)
{
	DefaultTextureList *dtxl = DRW_viewport_texture_list_get();
	DRW_shgroup_uniform_texture_ref(grp, "depthBuffer", &dtxl->depth);

You cannot read the depth buffer and test the stencil buffer in the same pass, it's undefined behaviour : https://stackoverflow.com/questions/36782796/read-depth-values-while-stencil-testingsame-texture

So a few workaround possible:

• Apply shadow in MULTIPLY mode in a separate pass. (not the best, performance wise).
• Split the composite shader to not use the depth buffer (that should be simple since it's only used to handle the background).

So I would suggest the 2nd option.