This was a few hours ago and i didn't remember it well when it happened so I'll rephrase what i sent to a discord server.

I was trying to select one ender chest out of a stack of 4 out of my endless backpack.
I right clicked to split the stack, then intended to right click again to drop the other echest back down.
I'm pretty sure that i double clicked(i can't recall left or right or if i dragged the stack at all) and the stack of 2 left in the backpack had disappeared completely, residing in neither in my inventory or backpack and not dropped on the ground.

I'm sure this is not a one-off incident because I've had miscellaneous items disappear on me with no explanation.

I'm in bed currently but will reply with the full mod list in the morning.

These mods were personally added to the modpack