Describe the bug
A clear and concise description of what the bug is.
There is a bug with the inventory. If you have an item in your backpack, and get more of that same item in your inventory and try to add it to the existing item in your backpack, the extra items you just added will disappear, leaving the backpack how it was before you added them. 
To Reproduce
Steps to reproduce the behavior:

1. Place any stackable item into the backpack's inventory.
2. Gather more of that same item into your own inventory or toolbelt.
3. Shift-click the items from your inventory or toolbelt into the existing item in the backpack.

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
If applicable, add screenshots to help explain your problem.

Required information (please complete the following information):

• Mod version: 1.16.4 - 5.3.7.jar
• Minecraft version: 1.16.5

Additional context
Add any other context about the problem here.

Use 5.3.8 instead