When I search for a term in a PDF, metapdf finds and highlights it. This prevents markup mode from working if the selection includes the body text that has matched the search.
If I then select text including the search match highlight in the PDF text body, I can't then mark up the whole block, something prevents it. I can select it, I can copy and paste the block (including search highlight text) as expected.
The bug is not serious but it is annoying and also results in the markup mode button appearing to do nothing. The work-around is to search again and clear the search box content. This removes the match highlight in the body. I also notice that when I clear the search text and the search text hit is within a selected block, it deselects the last word selected. This may be an out-by-one error.
Anyway this is a great product, and keep at it!