I had that overextension-flipping "feature" in my latest game too. No provinces or cores gained/lost, no change of ruler, no moving of the centralisation slider. I didn't have any overseas provinces either, so at least in my case that can be ruled out.
In Xnaop's case, the problem is that his capitol is in Sinai, which is in Africa. Provinces in Asia and Europe will temporarily become "distant overseas" if their path to the capitol is blocked by rebels, including the province(s) occupied/sieged by rebels. Since "overseas" provinces are either not counted, or counted weirdly, for the purposes of overextension, rebels will cause a fluctuation in the total number of provinces. Xnaop was probably just over the limit, and when he got rebels in an uncored province, causing it to become overseas, it pushed him just under the limit. That caused him to lose the modifier until the rebels were killed.
You'll also see the same thing happen with the colonialism CB on the borders of Europe, Asia, and Africa. If, for example, you have territory in Palestine or Syria and border an Asian Mameluk or Ottoman province that becomes occupied/sieged by rebels (or hostile nations, I think) You'll get a temporary colonialism CB until they retake the province.
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