I think I actually caused the issue when cleaning it in first place, as it was in pretty isolated area, just a 0603 resistor far away from the analog parts, it wouldn't have affected anything.
Probably the alcohol diluted it, making it runny and getting under capacitors, resistors, ics... then it dried and the moisture did the rest over time, I recall seeing the readings changing when breathing on the pcb.
Then I started poking with my fingers until I found the sensitive zones, flooded with alcohol and started brushing hysterically, probably the pcb is thinner in that area now

Perhabs a barely conductive path between an analog input and VDD, just microamps... but it was enough. No corrosion whatsoever, it's been fine for months now.
Is it happening in all resistance scales?
I rememeber about 15 years ago, I had a very cheap DMM.
Before measuring resistance I had to compulsively rotate the selector back and forth to clean the contacts, it was so cheap that they weren't gold plated, just bare copper, so they oxidized very easily.