Seen this as well on a M50/1.7. In their day, they typically had a skylight filter on the front to warm the images a touch. But I also found on digital that 30 year old skylights filters added odd tints, maybe the dyes in the filters deteriorating?
And yes, the M85/2 can go a little crazy with blue CA at F2-2.8 but it seems very neutral with colours overall which I imagine was a good trait for a portrait lens.
I switched to Raw to resolve M lens wb issues. The white balance is then just a parameter stored in the file and not baked into the image like jpeg. I found that when correcting wb, the tint often also needs a little tickle. End results can be good and worth the extra step.
M50/1.7 from Single in March 2013.