I got the same problem when using the built-in mariadb updater on Ubuntu 18
the config file is located at /etc/mysql/mariadb.conf.d/50-server.cnf
I commenteed the line bind-address = 127.0.0.1
Tried to backup and this fixed the problem... just adding my 2cents in case anyone else came with the...
I run intto the same problem and managed to fix it... so I kinda know how to reproduce it.
- Just replicate a wordpress website with this feature disable into a different existing one that have this enable... BAM, it gets disabled and you cant enable it.
To fix it: Enable search engine index...