Why (or does it?) unsubscribing remove the check?

Is it a "fault" of the software? In other words, I like to know (if I can't independently remember) whether I previously commented on a thread and I look for the check mark which is the telltale sign.

But it appears to be the case that if I unsubscribe to certain threads in a futile attempt at uncluttering my email box, the check mark is removed.

Well, if you've posted to a thread, whether you subscribed or not, the thread title has

HTTP response: NOT FOUND | MIME changed to 'text/html'

I didn't know that's what that meant. Ty. I was talking about an actual check mark you see to the right of the thread title in the list.

All that check tells you is that you're subscribed to the thread. So if you unsubscribe, of course it will go away. Hover over it on one of the threads you're still subscribed to, and that's what the message will be. Hover over the image I posted and it'll tell you how many posts you have in the thread, and when the last one was made.