End of the tin?

Good day!
I've been pipe smoking for about a year now and I notice a bit of an issue when I get near the end of a tin (approx. 25% remaining): it tastes burnt, flavor is gone, perhaps burns too hot. Really just completely unsmokable. Huge contrast, doesn't make sense to me.
I suspect the tobacco is getting dry at this point, but while it has less moisture to the touch, it doesn't feel THAT bad. I simply don't understand why the first half of my tin, which I usually smoke within two weeks, is beautifully flavorful, without any nasty charred notes at all, and then that happens, usually a month after opening the tin.
I should note that I've tried storing in a mason jar, as well as keeping it in the tin and putting that in a Ziploc bag. No difference -- when I get to the end of the tin, it's unsmokable.
I should also point out that I have a very delicate cadence -- I can't imagine I'm over-puffing.

I only smoke one tin at a time.
EDIT: I can sometimes go a week without smoking though. This was the case yesterday: I hadn't smoked in a week, pulled out my tin and it tasted disgusting!

I think it's probably a combination of the 'dust' as you put it, and drying over time as well as drying faster because its not compressed in the tin with the rest of the tobacco. I just don't understand why it happens at ~25%, and why it's such a huge contrast. I should try smoking a tin very quickly to see if time has anything to do it