Everytime I put items into pouches, backpacks or rucksäcke they appear there. But if I close and then reopen my inventory they are all empty

Can you provide a bit more information, I cannot seem to replicate this. Which version and forge version are you using? Did it happen in single player or multiplayer?

I am playing singleplayer with 1.12.2-14.23.1.2555 forge.

Rikmuld I got how bug works if it helps. If you put new crafted (dragged from creative) rucksack in slot without opening it first with RMB (and putting things inside) it will be buggy, otherwise not. The state of buggy/not buggy survives the game reload.

@warudo89 thanks, that makes it clear, easy to fix now :)