Agree - something is up with your power meter on that ride
Do you have power files from prior rides with that group? It sounds like you ride with them all the time - compare to last week or another earlier week?
What power meter is it? How is it connected (BT or Ant+)? The reason I wonder is that the latest Assioma Uno update does not double the power of the one leg on BT but does on Ant+. Any chance that something like that is going on?
Or maybe low battery? At a minimum I’d yank the battery, do a reset (usually putting a CR2032 in backwards), test battery, and replace if low voltage.
I think you may have solved the issue! I just changed my battery out last week and upgraded to a Garmin 530. Also, I checked the power file from the same ride Last two Sundays back on July 14th, the the NP was 206 with a TSS of 148.
I’m riding with a Pioneer meter that comes on Dura Ace 9100 gruppo with a Garmin, so it’s an Ant+ connection.
I had a Pioneer meter. It sounds like the battery in one side had died. Depending on which of its many modes it was in it might not double the remaining one side power.
I’d agree with STP. I’m always surprised at the lower TSS and normalized power on hard group rides compared to much shorter training workouts.
like that outside, chasing rabbits like a greyhound 
If it’s not a dead battery, then Saturday night you must’ve partied like it’s 1969.
I so agree. I’d rather be outside when I’m doing a lon hard ride.