Using Potion Core and Rustic together it makes ironberries allow you to do insane jumps because of the very high jump boost effect and how potion core modify how jump boost work, by making you jump high controllable so that you can jump 3 block with jump boost 10 for exemple. Ironberries give over 127 lvl of jump boost, (to make the player unable to jump) but potion core read that the player can jump 128 block high if they hold the space bar.

An idea to correct this bug is by making a new effect that would replace the jump boost effect, or give the player a high level of weight (which is from potion core) so that the player can't jump even with the insane jump boost

I am also experiencing this issue.

You can also turn off the controllable jump on potion core but a real fix would be better

I'm sure you've moved on since this bug report, but just wanted to mention RLTweaker, an in-house compatibility mod made for RLCraft fixes the Potioncore/Rustic bug with the jump boost. Granted, it makes you immobile now, instead of just unable to jump. I think that is a good compromise.

You can also turn off the controllable jump on potion core but a real fix would be better