Charm is a mod which introduces a lot of small things to the game.
One of the things is a Crate - a block which can store up to 9 stacks of items.
Here is an empty crate:

And here are two identical Crates with random modded block inside of it:

Crate itself doesn't allow Shulker Boxes inside of itself, nor Shulkerboxes allow Crates....
But Crates can be but into Backpacks, and Backpacks into Crates creating an infinite storage cappability:

Just like there!

I guess you can already see an issue and I don't have to explain it, right?

There is nothing I can do! This issue has existed in every single mod that has added any sort of storage through an item. It has existed since the very start of modding.