Recovery mode keeps reverting to full
SQL Server does not automatically change the recovery model.  If you set it to simple, then see later it is set to full, either:

Someone else changed it back to full
The database has been restored from a backup where the database is set to full recovery
There may be a scheduled agent job that is changing the recovery model.

From a comment on this answer:

This was in AWS and #3 was the case. In this case backups were enabled so they have a process that automatically changes the recovery model back to Full. Changing the backup retention to zero days allows it to stay in simple recovery.