Thank you so much for fixing it! Out of curiosity: Can you shortly elaborated on the problem and how you fixed it?
I just disabled the plugin “Disable Gutenberg” and I think this fixed it for me.
Yes, same here. The problem is that already existing custom permalinks get overwritten with the default link every time a post is saved.
You can see it as soon as you start editing a post. The custom permalink field is already reverted to the default link. If you insert your custom (old) link by hand and save, everything is fine. If you don’t, well, you just overwrite it.
Also happened after the WP 5.5 update.
