Probably a quirk of the OS Often difficult to fix.
This using Gimp 2.8.22 / Xubuntu 18.04 - the help setup in Edit -> Preferences https://i.imgur.com/v53T9QE.jpg
F1 takes me to https://docs.gimp.org/2.8/en_GB/gimp-image-window.html (obviously depends on locale but should be similar. The browser is slimjet - another chrome based one.
If you want a PDF manual for off-line use there is one here: https://gimp.linux.it/www/meta/gimp-help-2.8-en.pdf
Edit: According to distrowatch
By employing its Site Specific Browser, Peppermint integrates seamlessly with cloud and web-based applications
That browser might be the problem
This using Gimp 2.8.22 / Xubuntu 18.04 - the help setup in Edit -> Preferences https://i.imgur.com/v53T9QE.jpg
F1 takes me to https://docs.gimp.org/2.8/en_GB/gimp-image-window.html (obviously depends on locale but should be similar. The browser is slimjet - another chrome based one.
If you want a PDF manual for off-line use there is one here: https://gimp.linux.it/www/meta/gimp-help-2.8-en.pdf
Edit: According to distrowatch
By employing its Site Specific Browser, Peppermint integrates seamlessly with cloud and web-based applications
That browser might be the problem