This seems bizarre:
My typical process will be to keg most beers, and if I really want to get something from the fermenter to keg before a keg is freed up, I will bottle off the remainder of the previous keg with either a beer gun or a bottle wand, and rotate in the new beer.
In a few instances, I have noticed the beers have become OVERCARBED after aging in the bottles. My first thought would be infection, but I tasted a bottle of my 80/- from 9 months ago last night, and despite violent “psst” when opening and baking-soda-in-vinegar-like carbonation in the glass, the beer tasted fine once it degassed a bit.
One other detail: i will usually quick/blast carb for 30-45 seconds at 30-35 psi when kegging a new beer. The only other thing I could think of is when I carbed the 80/-, it actually may have been overcarbed, as at that time, I was turning psi down to 2-3 for serving (and bottling in this case).