Seems ever since I released the library no one ever tried to work with a limited number of channels on EU868. In default configuration most LNS enable the 5 additional channels via Cflist.
Now while desperately going through the library I found a very old setting (originally from Semtech), that always enables all 8 channels on EU868.
inside the SX126x-Arduino library folder go to
goto line 45
#define USE_SEMTECH_DEFAULT_CHANNEL_LINEUP 1
#define USE_SEMTECH_DEFAULT_CHANNEL_LINEUP 0
After that, the node will use only the first three default channels, unless the LNS enables the additional channels.
Tested against Chirpstack with only 3 channels enabled => using only 3 channels
Tested against TTN V3, which enables additional 5 channels after join => using all 8 channels
Want to do some more tests, but it seems that was the problem. Will push a new version of the SX126x-Arduino library by end of the week.