Thanks the replies. Just to give some more details:
It started when I got a warranty replacement frame from Giant and they put my original brakes/disc on the new frame. that store then looked at it 3 times and replaced the pads first then disc+pads and both times cleaned everything well. They couldn’t find any issue and said they were “satisfied” with it but it still squeaked so I took it to another store and they first have it a thorough clean and then replaced the brake caliper as a last check.

I don’t believe it’s solely contamination as it happens even on a new set of pads after the first ride. It only happens in colder conditions (I live in Australia so it’s not ever really that cold).

they are Shimano resin pads L050A-RF.
the disc is Shimano RT-MT800-S
the bike is a TCR Advanced SL

it never happened on my bike before I got the new frame.