Here I am in the Mod Queue of r/stupidquestions, looking at posts and replies that have been reported. At the bottom of the post or reply I have buttons to Approve or Remove, and a drop down menu to do some other things.

https://gyazo.com/b9de988724e8392fb8fbcc64a402771b

If I click on Remove, the Approve button changes to become an Add Removal Reason button (and the Remove button changes to an Approve button in case I made a mistake).

https://gyazo.com/49f343767a183b6cbc2bc8654b0a3a94

If I click the Add Removal Reason button to choose my removal reason, about half of the time I get a response that "You don't have any removal reasons yet."

https://gyazo.com/657e746c71f0bac2433904eb3b84eb1c

I know that our sub does have removal reasons, so in curiosity I click the link that says "Add a removal reason." That click takes me to the r/undefined sub.

https://gyazo.com/94d82ae20a581a705b572818f12d8d25

The other half of the time, I get a window with the drop down list of our removal reasons, and I can select one of them. I'm not sure why this keeps happening, and I wonder if other moderators are seeing the same thing.

Apparently something like this was happening two years ago (https://www.reddit.com/r/ModSupport/comments/vk3iqv/mod_queue_posts_linking_to_rundefined_instead_of/) and again one year ago (https://www.reddit.com/r/ModSupport/comments/y2otej/mod_queue_constantly_misbehaves_with_links_to/). Sorry to be the bearer of bad news.