I had this problem - I defrosted milk that was over 2 months old to start with, and it smelt and tasted vile (a mixture of just 'off' and also an odd paintstripper/solvent type smell).
Then I panicked and started working through my freezer stash, and it was only the milk frozen within the last ten days that smelt and tasted okay.
I was really gutted, and read about the lipase levels, same as you. But in any case, I bought a new fridge/freezer, as mine was old, and after putting the bath thermometer in the fridge section, I realised it might not be firing on all cylinders anyway, and the freezer section not freezing well enough.
In the new freezer, milk over a month old has been absolutely fine. I haven't any older milk to test yet.
So only practical advice I have is to check how cold your freezer is, and see if there's a difference between a very recent batch of frozen milk, and an older one.
I did read other stuff (on MN? Can't remember) about frozen milk tasting weird, but babies not minding. I decided mine tasted so vile, I wasn't 'testing' DS with it.
I also decided that I wasn't going to have a huge 'archive' of old frozen milk, but would rotate stock, so there was never anything over 2 weeks old in there. Partly so I didn't risk going out and leaving an 'off' bottle for DS, but also because if boob milk is so marvellously attuned to my baby's needs at various stages, then I wondered if it wasn't ideal feeding 2 month milk to a 4 month old, IYSWIM.
Sorry I can't be more help, I know it's very disappointing when you see there's a problem with your freezer stash 'safety net.'