The item's action is based on the slot that it was in when it entered the hotbar (maybe this is a non issue with the prompt for activated items). Moving the item to a different key does not update the alternate cast indicator.
Maybe hideout only? Instant cast still works poorly with alt cast binds like scroll wheel so I had to switch it back anyways x.x I wish there was an option to make items popups appear like instant cast does it without having to change cast modes though.