Something must not be working once you select a post. This is what you should see as soon as you’ve selected the Select Posts… (the top line indicates the number of posts you’ve selected and should change as you add more.
Once you select to move the posts you should be presented with this:
Maybe someone else can chime in here and suggest something. If you are still having this problem, something “broke.” You can try this is Safe Mode and see if the problem remains or not. Another thing to check out are Plugins. One may be interfering mid-way through this process. If you’ve installed a new plugin recently - before this problem arose - that could be the culprit. Otherwise… someone with a lot more expertise will have to suggest something different. Sorry.