Yeah, @hans1 is mostly right. He’s correct enough for your question/issue, but I’ll clarify one small detail…
Updates on March 30, 2022 (Release 536) changed the underlying data structure of activities in a way that made processing way faster and not based entirely on GPS coordinates but rather the overall path of the activity.
You can conceptualize this by mentally adding a 25 meter buffer around the line of your activity, changing its shape from a few pixel line into a kind of oval-ish blob that might look something like this:
So any nodes within that red buffer area would be marked as complete - it wouldn’t matter where on the blue line your GPS coordinates were actually placed.
In your case, the GPS signal cut out & a straight line was drawn between the point where it dropped & it reconnected. If that tunnel had been straight and coincidentally matched that straight line, then those nodes in between would have been marked as complete - because the system looks at your overall route to determine which nodes it completes.