tbh I don't think I would consider this a bug, nor would I know how to fix it.
This is kind of a staple of video games, if you hover a character or an attack right on the edge of 2 different states, a few things are going to happen.
the character/attack is going to quickly alternate between either state
the character/attack is going to pick one state and dead-zone or time-lock you out of the other state
We intentionally chose to go with option 1 so players have more control, but can get in a state where the game essentially tries to check state A or B every frame (60 times a second) - which can result in it changing on you.
So... just pick a side more decisively?
EDIT: it might be possible to make it so the "portal doesnt lie to you" but it is likely occurring due to the reasoning i provided and might not be possible to fix unless you'de prefer a quickly flickering blue/orange targeter. Sorry I could have been more specific and thanks to OP for adding the more specific comment.
EDIT2.0: OH! i forgot sometimes things exactly like this behave differently on jungle practice than in online games, due to additional server client checks that dont happen in jungle prac. I wonder if its better or worse in a real game...