I started to have this issue and I cannot figure out why. I have a OenPlus 8 on OOS13 latest version. AdAway 6.0.3 is unable to write into the host file. I am rooted, Magisk Canary on the latest version, I have Systemless Host enabled and I have Magisk built-in by @zgfg
Busybox module installed. Why AdAway cannot write to the host file is confusing to me. I have clear the data on the app and restarted it and nothing. If I delete the app and try reinstalling it I get failed to sync during the installation. Not sure what is going on.