All of the mods for 1.12.2 forge I have including BetterStorageToo:
DamageIndicatorsMod
Advent of Ascension
ArchitectureCraft
BiblioCraft
BiomesOPlenty
Chisel with CTM
ChiselsAndBits
VoxelMap with CodeChickenLib
GraveStone
Hwyla
Walia-Harvestability
Just Enough Items
Not Enough Items
McPaint
Natura
Optifine
Tinker's Construct with Mantle
WearableBackpacks

I think the bug is caused by my chest being too close to one another because the two big reinforced chests behind of me stack on top of each other are fine

edit: I tried giving the large/double chests some space, the bug still persists. they randomly split up during gameplay and contents inside are gone.

Today, both my friend and I had this happen to us in separate instances. He had several chests next to each other that all glitched out, and I had a double chest by itself.
Edit: I did it w/ lockers and it happened too

Seriously,it is kind of critical bug for such a storage mod. 💢
It is my favorite mod from 1.7,but now it just decide to throw all player's item into Other Dimension while it not sure what kind of chest it is or which chest it should connect to. 🙄
Every chests in this version have several critical bug, it just too buggy to play with. 🔥

I have the same problem with vertical lockers when putting them together. This happens randomly, I have many and it happens to some and not to others, but in the end it happens to all.
I can't always reproduce the bug but it usually happens to me when I enter into the server, when I get closer, they become transparent, and when I move, they rematerialize, but already separated and losing the content...

I attach some screenshots where you can see how it should be fine and how it is bugged:

If I can help you in any way, please tell me, let's see if we can solve it, because this mod is perfect for what I want.

How to reproduce @LaysDragon @VioletJoyFalls

How to reproduce @LaysDragon @VioletJoyFalls

I can't reproduce this bug anymore since server is close after long time ago. But as I remember it will happened while serveral big chests is putted together ,then it is easy to broken while chunk reload.