Unable to export marked comments or to delete comments

I have received a pdf-file of a book inset in return from my proof reader with almost 1,000 comments in it. I have gone through them and have marked 40 of them which I need the author to have a look at. I would like to delete the 960 comments that I have already handled, save a new version of the PDF with just the 40 remaining comments and send that off to the author. 

I am unable to do that.

For some reason the choice of deleting comments is greyed out. This applies to both single comments and if I mark several of them in a row.

Then I thought I would try to export the marked comments, which there is a function for, but this exports a file with ALL comments. 

Why is this so difficult to achieve? I cannot be the only one with this issue.

I am running version (build) 24.2.20759.0 of Acrobat Pro on a Mac Studio with Ventura 13.6.7