I tried reproducing this issue, but could not replicate it. I was able to copy text to filename using both CMD+v and 'Paste' option in right click context menu.
Can you check if the text is copied in the clipboard by trying to paste it in a text editor?
Where are you trying to paste it to?
I've had no issue pasting into the box either. You've definitely got the test box selected?
Custom Text gets added after the template has been created if you're trying to use something with that.
I verified that the copied text is in my copy/paste buffer.
In the screenshot I included I'm trying to paste the text between the 2 dashes - so I have:
[Date (YYYY)] - [Date (MM)] - [Date (DD)] - <trying to paste here> - [Filename number suffix]
I just found that if I paste directly after the Date (DD) block - before the space it works! Thanks everyone for the help, I wouldn't have figured that out on my own.
[Date (YYYY)] - [Date (MM)] - [Date (DD)]<pasting here works!> - - [Filename number suffix]
This still looks like a bug, but admittedly an edge case.