The same happens here... If I manually select new timezone, after seconds it changes automatically to são Paulo one (that is currently wrong because it has summertime and we are not having it this year) And option to use timezone from network is turned off... It started to happen after last upgrade to Android 10..in 9 it was working fine for me.
Playing around a bit more it seems that if I don't select both options (to have clock updated from network and to have timezone updated from network) , then it stops to change my timezone automatically... But it should not be like that... Usually I was keeping the clock update by network but I was manually selecting the timezone(because of bug with Brazilian summer time)