UPDATE: My Mistake - it did not correct itself. Blank record is currently the only solution that is working for me. I am going to try the Browser vs Custom workaround again (tried it after reading your post but didn’t get the desired result) and then will post a new independent request for help if that doesn’t work. I think we are experiencing the same problem but I believe my relationships between collections/datasets/filtering/dropdowns is slightly different than yours. Maybe if I provide the details it will trigger something not yet considered and move the system towards a solution (or educate me if I am the problem).
The problem seems to have corrected itself since having made this post. Without changing my code now the placeholder appears as expected after clearing the dropdown. However, in the meantime what I did was add a blank record in the table I was using for my dropdown list. When the blank record was selected then the placeholder would show. Not sure why it worked but it was a work around that solved the problem at the time.