Although it may not be IP's fault, there may be workarounds.

A comment somewhere implied that this may be due to a portal plane being rendered on top of UI elements. I've managed to enter a situation that strongly implies this, as hotbar items can be partially occluded:

So depth buffer issue maybe? Somehow gui getting confused for the portal?

dropping any "hidden" item causes all hidden items to reappear, temporary workaround?

Maybe fixed in https://github.com/qouteall/ImmersivePortalsMod/releases/tag/v1.3.4-1.18 (I am not sure)

probably fixed