I cloned your repo and modified your ext.php to install on my 3.2.1 test-bed (btw better to check for constants.php there since you are relying on future MCP's template events you did self-create 2 PRs for
I can't reproduce your issue as you can see here in my grabs I posted below, everything seems to work as it should.
You sure you selected "use best answer" creating the subforum? If not and then it works for you as well then you should consider to add some check into your above code - which seems to me a bit "poor" at a first glance (missing checks and conditions?), so to speak. More if you tell me what ever I could do to further test your ext.
On a side note, the FA icons looks like a "topic solved" icon, maybe because in this site it is used like that and I am used to - instead at stackoverflow means the the best answer.. in fact. Tastes.
May I suggest to change it maybe with a trophy of any sort?
Last but not least, IMHO that icon should directly lead to the relative post which has been marked as the best answer, if clicked. Along with an ALT/TITLE attribute, that would be an excellent enhancement.