Any items that have a modified behavior "stack" size limit in the "items" folder  (including both vanilla and custom items) have issues on realms only. Please note, this was tested on local devices with 0 issues. Its only happening on realms.

Steps to Reproduce:
1. Download the addon linked to this report, or any addon with an item (custom or modified vanilla) whose maximum stack size is below 64
2. Add the addon as a behavior and resource pack to a realms world
3. Move it around the creative or survival inventory, shift clicking and pulling stacks out of inventories, and merging them

Observed Results:
Stacks flicker with a full stack size before the excess disappears. When merging 2 stacks already in the inventory, the excess disappears. The overall experience is glitchy and results in voided items or errors.

Expected Results:
Stacks smoothly merge or are prevented from merging, and are moved around the inventory while preserving their maximum stack size.

Additional Notes
If you have a pumpkin pie item that has been modified to just 1 item allowed per stack, but have 5 other "stacks" of it in your inventory (that are also just 1 item per stack due to the behavior pack), the pumpkin pies will be deleted if you try to "quick move" it from say your inventory into a chest. Leaving you with just 1 pumpkin pie, and 5 missing ones. Gone.  This occurs in villager trades, chests, player inventories, etc.

We've tried downloading the realm files on multiple devices and each works flawlessly until realm is introduced. So its not a cache issue, nor is it a coding issue with our json file. I have already brought it to CornerHards attention in one of our multiplayer discords as well.