I'm not sure what it's scaling to (something off leader's cp?) but IF you are already in a group and directly port into a dungeon with it set as vet, it seems to be scaling enemy resistance higher than if you were placed into the dungeon using the group finder queue. I seem to remember that used to be the way it worked backed in the day, then they eventually changed it as a low level lead allowed you to burn it fast. However, even last year I remember this being an issue where all of a sudden resistance seemed to increase, and remember FGII specifically when it was bugged and you couldn't even burn the adds down that chain your limbs fast enough as their resistance was too high.
Being dps and running same gear for awhile, you can tell how quickly mobs/bosses burn or don't burn (like the difference between normal and vet mobs, etc.) i.e. what's normal and what's different from buffs of the group, etc.. And with the aforementioned FGII bug from about 1-2 years ago where the resistance was abnormally high, it was clear fighting anything what was going on.
And to be clear, not saying "oh I wiped and want an excuse" it's enemy resistance seemingly going higher than intended and spending an hour fighting an easy vet dungeon because you ported in directly vs. group finder scaling where you burn it in 20 minutes or whatever.