I have the very same problem here. In the begining changing the default font worked, but after a reinstallation of the operating system Adobe Acrobat 2020 always defaults to Helvetica, no matter what I pick as comment font. For me the problem is quite severe though, as I normally write comments in non latin letters. Helvetica does not support those characters and Acrobat then seems to switch to the next available font that supports the character. This font however does not support all the characters and as I write the comment it will switch the font again. So my comments are mixed up with three different font types. It looks unprofessional and I need to manually change the font for every single comment. That's quite annoying!
I have the very same problem here. In the begining changing the default font worked, but after a reinstallation of the operating system Adobe Acrobat 2020 always defaults to Helvetica, no matter what I pick as comment font. For me the problem is quite severe though, as I normally write comments in non latin letters. Helvetica does not support those characters and Acrobat then seems to switch to the next available font that supports the character. This font however does not support all the characters and as I write the comment it will switch the font again. So my comments are mixed up with three different font types. It looks unprofessional and I need to manually change the font for every single comment. That's quite annoying!