Modpack version
1.59a

Can the issue be reproduced?
Yes.

Issue
Sometimes pressing any key will move items up one row in inventory.
It happens 80% of the time I played. I don't know how to constantly reproduce it.
I've checked with key binding, found nothing.

Please help or at least give some clues, thank you.

This issue is usually caused by the Cyclic keybindings to move the hotbar, they can act weird when unbound. I'll report it to the mod author asap :)

Lothrazar/Cyclic#972

I've experienced this with English input, but thank you for the extra info.

FYI, I'm a Chinese user and it mostly happened when I had mandarin input method enabled. Currently my way of fixing this is switch back to English input method and do an alt+tab, everything will be fine then. And I haven't experience this issue on other modpacks.