Never mind, I figured it out:
If .p2k.cfg only partially works on C64: Maybe parts of it get overridden by the C64 emulator settings. To check (On a GPi case):
- Select B to open Quick Menu
- Options (select with B)
- Retropad Mapping Options
- Remove conflicting entries (set to
---) - back → back to Quick Menu
- Overrides
- Save Game Overrides