It would come from the configuration of the Database formatting, schema, etc that, if incorrect, would hinder the proper operations of Mattermost even while it is running. I have been referring to this Mattermost Github issue, which seems to be exactly what you are experiencing. If you read it from top-down, you will see me asking about the schema of the table is related to the first posted possible solution in the Github thread. At this point in time, I am now asking about this possible solution, which from what I can see, is what your configuration is set to be:
cedarice commented on Feb 28, 2019
I found a solution:
For mariadb, update your *my.cnf file with following configuration,
innodb_default_row_format=dynamic
innodb_file_format=barracuda
innodb_file_per_table=true
innodb_large_prefix=true
Then, you have to restart mariadb service for updated configuration to take effect.
reference: https://stackoverflow.com/a/52778785
If you take a look at that, could you let me know what you find, from the various schemas that you may want to check, as well as from your configuration files?