This was found in Darkosto's Sky factory 5 modpack.
1.20.1 - Minecraft
2.1.2.7 - CarryOn
1.0.1.809 - Sophisticated core
1.0.2.985 - Sophisticated Storage
When you pick up a sophisticated storage double chest(Seems to matter which side, I had the "best" luck with the left chest), put it down, pick it back up and place it again, it deletes the "second" half of the items in the chest. Vanilla chests work just fine. Darkosto recommended reporting this to you instead.

Showing Inventory

Showing double chest.

Showing probe of chest after picking up once.

Picking it up again and showing probe of lost items

Showing empty Inventory.

Possible solutions?
Maybe having the chest truly split when you pick it up, instead of this half split.

I will see if this can be solved somehow, but it's very likely the only "solution" will be to mark chests as immovable by carryon. Looks like Carryon doesn't give me a chance to move the items to the second half which logic actually is included in the code, but it's also super annoying to work out anything concrete here because I can't get Carryon running in my dev environment without revamping the whole thing and migrating back to forge and hoping that will make carryon not crash in there.

I will see if this can be solved somehow, but it's very likely the only "solution" will be to mark chests as immovable by carryon. Looks like Carryon doesn't give me a chance to move the items to the second half which logic actually is included in the code, but it's also super annoying to work out anything concrete here because I can't get Carryon running in my dev environment without revamping the whole thing and migrating back to forge and hoping that will make carryon not crash in there.

Fair enough! Thanks for being willing to look at it either way, sorry it sounds like such a pain in the butt.