When I use the editor for 10+ minutes, moving and replacing tiles around, some tiles eventually become unable to be replaced unless they are manually selected and removed. I haven't found a quick way to reproduce it, but it happens naturally after using the editor for a while.

The error that appears at the bottom of the editor is this:

WARNING com.threerings.tudey: Attempted to replace existing entry.

followed by other specific text. I've taken screenshots of it: https://imgur.com/a/eyPKM2v

So far, my workaround has been to restart the editor.