I'm having the same issue and at the same time hearing "pops" when it is printing, so was leaning toward it being a water saturation issue. Yes, I know it doesn't make sense that it is more brittle, but the sound it makes when printing seems typical of moist PVA. My spool of PVA is older (3-4 months), always kept sealed in a plastic bag with desiccant, in a plastic storage box with desiccant, and only the last few meters of the spool has this brittle issue.
Last print I just let the broken bits stay in the bowden tube and pushed it through with more off the same spool. Sure, I didn't get retraction, but it worked well enough for what I was printing............