No AUX channels on a Futaba S-FHSS receiver

I've just got this small quad with a Futaba receiver. I'm using a Devo 10 with this build . It binds (steady green on the receiver) with the S-FHSS protocol.

Sadly, I realized that only 4 channels are available. So it flies, but I can't configure an arming channel or a flight mode switching channel, because all AUX channels up to Betaflight's max_aux_channels are stuck around 1520 and the remaining AUX channels show 1500 all the time. I tried to reinitialize + rebind, change the number of channels in model settings, change max_aux_channels in Betaflight (1, 2, 4, 6), reflash to Cleanflight and repeat, but nothing helps.

Other than the common freq fine test (which yielded -14, a freq fine value between the two extremes) there are no extra settings. I just configured my Devo 10 so that it has some inputs assigned to all the 4 extra channels, just for testing, but no luck, only AETR works and the 4 extra AUX channels are simply dead on the flight controller side.

Apart from the tiny Futaba quad, I have e.g. XK X251 and XK X252, both with S-FHSS receivers and both configured on the same Devo 10. They use at least two AUX channels without any issues. (Sadly, they don't have an open-source firmware, so I can't explore how they are configured.)

What could be wrong on the small quad? I'd like more channels and I'm out of ideas.