myrocks uses dynamic or server assigned plugin ids. Due to some bugs in the server code, this causes problems with upgrades where the plugin might be assigned a different id from the one tables were created with. The plugin id is stored within the .frm data of the tables.
This was an issue with TokuDB where if a table was created with one plugin id, and the plugin id later changed, a DROP TABLE would never get to the storage engine. So the server would drop the table, but it would never get dropped at the SE layer. There were other anecdotal reports of issues similar but this one was specifically very tangible.
Recent resync to upstream reveals they also changed the ID, but it overlaps with our current ID for TokuDB. We're bumping the ID in Percona Server as we do not expect to support binary cross grades from FacebookSQL/MariaDB to Percona Server.