<p>I know that it's said the PLA becomes brittle when kept in a humid environment, but my case is slightly weirder:</p>
<p>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.</p>
<p>Is it still the humidity that somehow is better kept inside the tube, or is there a weird reaction of PLA with PTFE?</p>

I have this exact problem as well. I am feeding filament from the dry box through the tube into the top of the hot end. After approximately 2 days I would notice that the filament is broken somewhere close to the hot end. I don't believe that it's just humidity.
I suspect that the filament being brittle and being made to conform to the PTFE tube's shape, having been recently on a spool, is causing the breaks.
As a result, I'm going to write a small script that will heat the hot end and eject 200 mm of filament every so often.
EDIT: Whilst the script works, the filament would just break somewhere else lower down in the tube. Therefore the amount of filament that would need to be extruded periodically would be just wasteful. A better solution would be to unload the filament, either manually or automatically (somehow).