Items inside Inventory Upgrade get wiped on opening the server to LAN

[DecemberDumpling]

Minecraft Version: 1.12.2
Forge Version: 14.23.5.2860
Mod Version: 1.20.12
Single Player or Server: Single-player
Describe the problem (what you were doing; what happened; what should have happened):
I lost an item and wanted to cheat it back, so I opened to LAN to use /give
only for my items in my Extended Inventory to vanish.
This happened multiple times, until I finally realized the cause.
Might be linked to eating the upgrade cake multiple times? Who knows

Log file link: I don't think it created a log?
Video/images/gifs (direct upload or link):
https://i.gyazo.com/f79b2e5e2c4319a16545541c56a98e82.mp4

[DecemberDumpling]

This apparently happens cuz your UUID changes when you open to LAN