I had this occur on two batches.
I narrowed it down to something funky going on with some O-ring on the keg.
I relpaced the O-rings (all of them not just the big lid one) on the keg and tried a third batch and this one was fine.
If its been the same keg, I'd suggest changing all the rings and see if that helps.
Carb some water to test so you don't waste more nectar.