It's definitely a 5G thing, I've never thought of that, will need to research why this happens at the end of the RF.
I've always been reluctant to LNBFs, specially in C-band, prefer good old feedhorns with fit-on LNBs. I do understand that these 5G filters/LNBs simply cut off everything below/beyond certain frequency and that is out of question for me, I still receive some russian & nepali signals below 3.625 GHz more or less uninterrupted and everything between 3.810 GHz-4.200 GHz as well. It's just this narrow range above 4.160 GHz that bothers me. Considering 5G towers from three different directions (closest being just 300m away) maybe I should simply be happy to receive something at all...
Maybe a simple inline atenuator would help?!