A BIOS module's glue.c includes overlay_dispatch_preamble.c.inc and psx_bios_module_glue.c.inc by name. The tcc path compiles against a BOM-stripped copy of the include directory, and that copy held only *.h, so every tcc module build stopped at "include file 'overlay_dispatch_preamble.c.inc' not found" and the runtime fell back to the bundled BIOS.
Copy .inc files into the tcc include copy as well. Game overlay shards were not affected: compile_overlays inlines the preamble text instead of including it.
Alexandros Mandravillis