Why do custom weapons drop durability between chapters?

Yeah, it’s a bit of a problem. For example, in chapter 5x, a character has a unique weapon with 70 durability. In chapter 8, it is significantly less than what they ended with after chapter 5x. This happens with another unit earlier on, having a 90 durability weapon, and even though I’ve never used, before I know it, it’s down to 28 or something.

By default, save data can only handle weapons having up to 63 uses. Any weapons above that lose 64 uses on reload.
The simplest solution is to not give weapons such high durability, but otherwise, look into Expanded Modular Save.