Remove commented highlights

Hello!

This has been dealt with before in a few threads. However, the suggested and accepted solutions there don't work for me. I have received a commented PDF and I want to remove some which are not needed.

As for the document probably being flattened - it's not. I can highlight in the text and simply delete my highlights, but not the alien ones. Weird is that Acrobat shows my commented highlights in a slightly different way than the alien ones.

What I tried:

1. Right-click on the comments and choose Delete -> that would remove them as comment in the comment list, but the highlight and the comment symbol are still there, only they're not highlighted as comment anymore with the blue frame

2. Because suggested in another thread that Edit mode could help, tried Edit mode, but there are no boxes hovering the text.

3. Saved the document and opened again -> the previously deleted comments were back

The worst is that if the mouse pointer hovers over the highlighted text that has been removed as comment, it would show a tooltip with the comment. That's quite illogical. So far, there seems to be no chance to remove the comments. Any other idea?