It is possible that they had an engineer work on a fix in that (albeit narrow) window of time from your seat change to doors closing, during which time they could have reassigned the seat once fixed.

It’s also possible that there is a residual issue with the seat that is not apparent and it was given to someone on a staff ticket perhaps. I have found that the reports and notes on seat faults aren’t always accurate.

I would likely be frustrated too but knowing this also be mindful of some of the unknowns.