SOLVED! So the issue proved to be quite an obscure one. After a lot of troubleshooting, deleting a single entry from the wp_usermeta table fixed it: “meta-box-order_swfd_lesson”. Thanks Patrick!
Note that there are other “meta-box-order_X” fields that could cause issues in other cases. Also note that relogging into WordPress is required for the deletion to visibly take effect.
I’m not sure if this is anything that can be improved on any of the involved plugins’ sides (or that it’s a Gutenberg issue), but at least it’s a known issue now.
