I only just solved that puzzle today, and I didn't switch tabs or do anything unusual while solving the puzzle, since I was solving a bunch of them in a row. I suspect it has to do with leaving my tab open, because I discovered your game about two days ago, and that approximately lines up with the 2600 minutes.
Here's the relevant entry from the local storage:
"2026-01-28":{"isComplete":true,"startTime":1769739097515,"endTime":1769739059925,"completeTime":156463840}
Not sure why the "completeTime" seems to be unrelated to the difference between start and end times.