It was ‘fully working’ but reverted to a slower timing on transfers to maintain reliability.
Reseating it fully allows the slot and card to try the handshake at the highest speed again, though if it has the same failure (possibly a ‘power saving’ feature which is bugged in driver, a voltage sag due to dust/grease, possibly a vibration loosened connection) it may negotiate back down to x4 or x1 as it was in the beginning here. (PCIe x16 3.0 @ x1 3.0)
I’m not sure what the fault is, or why reseating the card fixes it, but I have seen dozens of reports of this sagging of the bus transfer rate, all fixed by physically reseating the card. This also applied to earlier generation cards, and I know of one ‘dying card’ 980TI with no fan spin up and iffy performance being fully ‘fixed’ by physically moving the card. (In this case it was to a different slot, but the ‘original slot’ functions perfectly with another card - it wasn’t my system to try putting the 980 back into the original slot to confirm a reseat would have been sufficient, but it does appear that the attempt to reseat a card fixes some types of problem where that seems improbable as the solution, and the attempt is probably worth the effort in most cases).