So, I’ve been having an issue with this feature since at least last patch, if not also the hotfix or two before then.
Whenever I start a whisper with one of my friends, I frequently put it in a separate tab soas to isolate it and not clog my normal chat window. But lately, when I do ‘Move to whisper window’ the new tab forms, but the whispers fail to relocate with it. When I try to continue, they’re still in the main chat window.
I don’t have any mods that effect the chat window, so I’m kind of confused why it’s acting up so bad.
I’d obviously recommend backing up your addons, especially your RP ones, before performing a UI reset, but that does seem to be the best option to start with.
An actual UI reset, like the one linked above, is a bit more than that - you’re thinking of a UI reload, which can also be done with /reload.
The reset requires uninstalling addon managers and renaming files - if you don’t back up your addons first, you’ll be missing all of the information on them.
You most likely have already checked your Settings, but just in case.
My friend had this issue the other day as well. I had him check the settings under Interface - Social. Some of his settings had been changed. The settings should be: