I am investigating more and have found that the problem is that many DVB-T muxes fail scanning in TVHeadEnd finding zero services, while using the USB pen I have the same muxes work perfectly.
The strange thing is that if I use the DVB app installed in the onboard Android, even with all its weaknesses (primarly, many out-of-sync issues), the DVB-T scan using the internal tuner finds all the channels that I don't find in Libreelec!