Well, I just downgraded to 18.04.1 (indeed reflashed twice, considering maybe something went wrong the first time) and I get other bugs 
- the ‘android app optimization’ process halts midway
- immadiately after I get two complicated messages, one of them concerning ‘android.process.acore’, which reiterates some 10 times
- these messages reappear regularly (when launching an app for instance), and are definitely more intrusive than the reboot issue…
- the updater app refuses to start (‘must belong to the system’ error… is it inherited from the previous and more recent build??
- when plugged to the computer USB I get an infinite loop of restarts never reaching the blue-bacground Fairphone logo
… I am now reflashing 18.09.2 just to see…