Missing Upload button on Insert menu for a specific user

One of my users reported that she is missing the Upload button on the "Insert > Image/Video/FIle" menu. This appears specific to one forum thread but does not occur for her when creating a new thread or replying to a different thread. I also do not see this issue when impersonating her. Here is her view:

When I try as her, here is my view:

Any ideas what can cause this? Permissions don't make sense since it works on my end and for other threads in the same group. She tried the usual troubleshooting steps -- clear cache, try incognito, log out/in. 

I've had other reports of similar behavior in the past, but it seems to just "go away" eventually. I'd like to have a more consistent experience for users if possible though.