I just found out that channels 8 to 11 don't output anything on my Scherrer v5 RX.
To confirm that it wasn't a faulty RX, I tried a Scherrer v6 RX and had the same problem.
To confirm that it wasn't a TX programming error, I tested the same TX model setup using a FrSky TX and RX and there channel 8 worked fine.

Has anybody had this problem or know the solution?

Some other possibly relevant details:
I'm using ersky9x firmware (= er9x for sky9x board).
Model setup PPM settings (defaults):
Proto PPM: 8CH 300uSec
Start Chan: 1
PPM FrLen: 22.5 mSec
PPM2 StartChan: Follow
PPM2 Channels: 8CH