Right, so I have spent a lot of time setting up functioning settlements with supply lines and for like 80 hours, things have been working great. But now suddenly, some settlements are glitching out. Settlements who have been connected to my supply network for a long time suddenly won't use the shared building material. When I try to build something it initially shows the proper, shared inventory but when I try to place it, it reverts to some local only inventory that doesn't have the required materials. I've noticed this in three different settlements so far.

Anyone else experiencing this? Know how to fix it?

I just experienced this same thing yesterday for the first time. It was kind of bouncing back and forth. It'd show all of my supplies from the connected supply line, and then flash to just the local supplies. Fast traveling away from the settlement and then coming back seemed to fix it for me. I've only experienced it one time. I was at Kingsport Lighthouse when it happened, for whatever it's worth.