My first Acetaldehyde brew

I've been brewing for a while now but have just recently brewed with Kolsch yeast. It's BM's Helles Bells.

I pitched 1.5 liter starter and fermentation too right off. It was temp controlled from the start at 66˚F. Maybe I should have gone to 2 liters in hindsight. BM's recipe states 2 weeks primary fermentation but I went for nearly 5 weeks due to work and family responsibilities. It finished a bit high.

I cold crashed for a week and rechecked my gravity. My initial taste before kegging was OK. Now, after nearly 4 weeks in the keg it has a noticeable taste of Acetaldehyde.

What should I do? Right now, I am about 10 weeks from brew day. I saved the yeast. Should I let it go and see what happens? Add yeast to keg and warm back up to fermentation temp?

Normally I would RDWHAHB but the Acetaldehyde taste is pretty strong.

Yeah, I just had the same thing that is either infection or more likely that the seal broke during fermentation and I didn't check for a few weeks and its from oxygen.