I have the same problem now!
Made some research on the Internet and it looks like it doesn’t affect only Fairphone. Someone on a forum says that Google messed up:
There is no fix because this is not a rom issue or device issue. Its a google mess up when they built 8.0/8.1 and word is it wont be fixed till later this year. Not all devices are effected but most are. I had to go back to 14.1 because the drain was massive. Cell standby was at 68% on 15.1 but is 1% on 14.1
I have read a workaround that suggests to set preferred network in “Phone information” (dial *#*#4636#*#* / Phone information / set preferred network). Some people set it on “LTE only”, others on “LTE/WCDMA”).
Any thoughts on that?