I haven't seen this problem with any other characters, only when commenting with Cyrillic letters and with word wrapping.
Not sure if this can help in tracking down the cause of the problem, but I've tested emacs 27.1 on a different computer and I've managed to replicate the issue. However, it had a clean Manjaro install (which is Arch based distro, and I assume they use pretty much the same libraries). Also, I've tried a number of other distros in VirtualBox (Fedora, Kubuntu...) and I've managed to replicate the exact problem on each and single one of those virtual machines.
When I downgraded to emacs 26, to test the issue, I was using the same system libraries like with emacs 27. And, as I understand, the only difference is with cairo, harfbuzz (emacs 27) and libmagick6 (emacs 26).
Also, I have experimented with different GTK settings, with font antialiasing, hinting, sub-pixel geometry, but nothing helps. The issue manifests even without X11 started, in plain command line mode.