Precompiled header makes custom-target-regenerated header trigger recompilation of all sources

[nyorain]

I'm building a C/C++ library with a precompiled header. Some of the cpp sources of the lib depend on generated headers, but none of the headers (including the pch) do. Every time any header is re-generated, all cpp files are recompiled, even though most of them have clearly no direct or indirect dependency to the generated headers.
This does not happen when I don't use a precompiled header.

To Reproduce
See a minimal repro here: https://github.com/nyorain/meson-bug-repro

project('buggy', 'cpp')

# very simple generated header
generated = custom_target(
	'generated.h',
	output: 'generated.h',
	input: files('generated.h.in'),
	command: ['cp', '@INPUT@', '@OUTPUT@']
)

# only bar.cpp depends on the generated header
shared_library('blub',
	sources: [files('main.cpp', 'foo.cpp', 'bar.cpp'), generated],
	cpp_pch: 'inc/fwd.hpp')

Every time you change generated.h.in, all cpp files are recompiled. Removing the cpp_pch line fixes that, then only bar.cpp gets recompiled on header change.

Expected behavior
I'd expect only the cpp sources that depend on the newly generated header to be recompiled.
In my case (the generated headers being shaders compiled and transformed into cpp arrays) this is very annoying and prevents me from using precompiled headers (since I have to recompile the whole lib every time I change a shader).

system parameters
On Linux, GCC, native build, my target is a shared library. Might affect other systems/configs.
python --version: 3.10.7
meson --version: 0.63.2
ninja --version: 1.11.1