<p>I suspect this is a confusion.  Here are some screenshots:  </p>

<ol>
<li><p>This is what I see on a question from someone other than me who hasn't accepted the answer:  </p>

<p><a href="https://i.stack.imgur.com/tWJW6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/tWJW6.png" alt="enter image description here"></a></p></li>
<li><p>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:  </p>

<p><a href="https://i.stack.imgur.com/ehoXm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ehoXm.png" alt="enter image description here"></a></p></li>
<li><p>These two are what I see when I am the author of the question, and I have accepted an answer:  </p>

<p><a href="https://i.stack.imgur.com/GRLpe.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GRLpe.png" alt="enter image description here"></a><br>
<a href="https://i.stack.imgur.com/0VfcA.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0VfcA.png" alt="enter image description here"></a></p></li>
</ol>

<p>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 <em>could</em> of course, but no one else can&mdash;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.  </p>

<p>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.  </p>
