This issue started about a month or two ago. I copy some text in the PDF (usually a title, or copy to use as a description for an alt tag) then I try to paste it in the properties box or as an alt tag description and it's blank. However, if I go and paste what I have copied into a notepad it pastes fine, so it did copy what I asked it to copy. Once I paste it into notepad, I can copy it and paste it into wherever I want in the document, so it is not a security issue with copy and pasting allowed.
This morning I had to copy a title written in all caps and paste it in a Word doc to easily change to title case. Then I copied it from Word and tried to paste it into the title properties box, but it overwrote what was there and left everything I had highlighted blank. So I copied the title case text from Word, pasted it into a notepad, then copied it from the notepad and pasted it into the title properties box and it worked.
I'm using the Ctrl + C and Ctrl + V for copying and pasting.
I have Windows 10 and the Acrobat DC version 2021.005.20058.