There are many tvheadend users out there and no one reported such an issue and there hasn't been any bigger change for this for a long time.
You can check the signal strength on the console and even tune on the console using "mediaclient"
/opt/bin/mediaclient --readsignal=0 -d /dev/dvb/adapter0/frontend0
/opt/bin/mediaclient -m DVBC -f 123000000 -M Q256 -S 6900000
-f 123000000 ... replace this with a transponder frequency
-M Q256 ... replace this with a transponder modulation Q64/Q128/Q256)
-S 6900000 ... Symbolrate
You can check if tvheadend is accessing the driver by running following command:
/opt/bin/mediaclient --lc
You might look out for hardware issues I do not think there's a software issue on your side.
Maybe even the BananaPI not having enough power somewhere.