Minecraft 1.8.9
Inventory Tweaks 1.60-40

Sorry if this is a duplicate. This happens with certain items from multiple mods. As an example, install Thaumcraft 5 (Minecraft 1.8.9) and give yourself any of the rings. (ex: Apprentices Ring of Perditio). Upon sorting, The sort stops halfway through and the ring is placed in my hand. Ring of Perditio is item #4265/8. Apprentices Ring of Ordo is $4265/7. There are at least a handful more of these, presumably all sorted as variants under the same number.

Placing the ring in a chest, then sorting my inventory again, then works correctly.

I'm guessing it might have something to do with how the mods add the items to the game, but whatever it is it's annoying. I would appreciate if there's a way to fix this.

Are you sure the option for server-side assistance is enabled, and you are in single-player or the server you are playing on has the mod installed?

That should fix most of these sorts of problems, since the usual cause is the server refusing to move an item correctly.