I'm having a similar problem in selecting an EPG lineup for the Telly tuner. I'm on the 2020.01.01.0239 build.
When I add the tuner and click the pencil to map the channels, it's already matched to a load of American channels for some reason. I click the plus icon, select the correct lineup, and then I have to manually re-map almost all the channels away from the American ones to the correct UK ones, even though the channel numbers from Telly all match the chosen lineup. (There are a handful that don't have an initial match with the American channels and automatically pick up the correct mapping to the UK channel.)
If I then close the mapping screen and then go back into it, the lineup I chose has disappeared and I'm just left with 'DVR: X-TELLY' and the + sign. My manual re-mappings remain, but they have lost the channel number. Any channel that wasn't manually re-mapped is now missing its mapping altogether. If I add my UK lineup again, those re-appear, but the same problem happens again as soon as you close the mapping screen.