Reloaded Meson project has wrong paths

When a Meson project with a some subdirs is re-loaded from a saved session, the paths to some files in subdirs are wrong. Wiping the project and recreating solves the problem.

I'm not sure how long this has been happening.

To reproduce:

1. clone https://github.com/opendcdiag/opendcdiag
2. configure (may require to be on Linux)
3. switch sessions to something else or close the project
4. switch back or reopen the project

Expected behaviour:
All source files are correctly found.

Actual behaviour:
Files with the wrong path are loaded in the project, resulting in their not being found.

See the file listing:

If you hover over one of the files greyed out, it shows a full path with "opendcdiag/framework/<filename>" with the "sysdeps/win32/" portion missing.

Also note how it's showing the Windows files on Linux, which can't be right. That may be caused by:

sysdeps_dir = 'sysdeps/' + target_machine.system()

if target_machine.system() == 'cygwin'
    sysdeps_dir = 'sysdeps/windows'
endif
...
if target_machine.system() != 'windows' and target_machine.system() != 'cygwin'
    subdir('sysdeps/unix')
endif
subdir(sysdeps_dir)