If a response (answer) is flagged as answering an OPs question ("accepted") and that decision is subsequently reversed, how does this happen? Who has the authority to reverse an answer? What are the typical reasons for these reversals? How often does this occur? Is a record of this change kept somewhere?
I suspect this is a confusion. Here are some screenshots:
This is what I see on a question from someone other than me who hasn't accepted the answer:
Here is what it looks like when there are two answers to a question from someone other than me, when the OP has accepted one:
These two are what I see when I am the author of the question, and I have accepted an answer:
So the point here is that when you are the OP, you will always see the check mark, whether you have accepted that answer or not, or even if you have accepted another answer. Only the OP can accept an answer / click the check mark. (The SE developers could of course, but no one else can—including the moderators.) Moreover, the OP can always switch to accepting a different answer, by clicking the check mark for a different answer, or can simply un-accept an answer by clicking the check mark again. The only thing you can see differently (when you are the OP), is the color of the check mark, which may not be very salient.
My guess is that you missaw or misremember the check mark as having been green before, even though it wasn't. Another possibility is that there was a weird and isolated bug that caused only that question to be displayed incorrectly for just you. I have no way of ruling that out, but from a Bayesian perspective, my prior is very low.