There have been reports in the past of a glitch on the DL App that let eligible passengers select their F seat before the upgrade actually cleared (I know this exists, I've used it before).

I've always had the status to be eligible for that upgrade anyway so I can only comment on that situation (e.g., checking at T-120 + 10 minutes and being able to move up before I would've been swept up at T-120 anyway) but I have never seen reports of that happening for non-status passengers.

Were you on a corporate ticket? That's the only thing I can think of -- I think corporate travelers are now eligible for upgrades as long as they are SkyMiles members but not sure (again have only benefitted from this as someone already with status so just get the added checkmark on a corporate ticket for higher priority over another PM without it)