It may be possible, that missing indices may block the updates from appcenter.
My last three Nextcloud - updates ended with getting stuck in maintenance mode.
I always could finish the upgrade with the three instructions I posted above. After that procedure my Nextcloud started and ran properly. At every update new indices had been added.
I recommend to run the updates first and then finally turn off the maintenance mode to prevent from client logins, which may lead to inconsistency. - I believe, that there is a special reason, why these updates are done in Nextcloud while maintenance mode is on.
Hint:
Please also check the Nextcloud logfile:
If Nextcloud gets stuck in maintenance mode, the loglevel might be in the wrong mode and the logfile becomes flooded by messages (several GBytes per hour).