I know that it's said the PLA becomes brittle when kept in a humid environment, but my case is slightly weirder:
I have rolls of 1.75 mm PLA that I bought years ago and they are fine. But if I leave my spool fed inside the PTFE (Teflon) tube of my 3D printer, that part that is inside the PTFE tube, and only that part, gets brittle. The filament spool is always (even when stored) in open air.
Is it still the humidity that somehow is better kept inside the tube, or is there a weird reaction of PLA with PTFE?