If the filetype of `xx.foo` is manually set to `C` the symbols are parsed, and at `(1)` (well the line after) `xx_<ctrl>space` correctly shows `xx_foo` and typing the `(` shows the calltip correctly, so tags parsing seems ok.
So its more likely in the Geany code of auto-autocompletions and goto symbol definition (which doesn't work in `xx.foo` either, not just `something.c`).