Setup:
Build a nether portal and start punching it in survival by holding left click. While punching the portal, go through it while still holding down left click. Once you go through, you are no longer punching the portal (or any other blocks you might be facing), even though you're still holding down left click.
Expected Behaviour:
You would continue to punch blocks until you release left click, even when you move between dimensions.
Alternative Demonstrations:
This also occurs with right click, which can be tested by giving the player a stack of honey bottles. In survival, stand in the portal and start drinking the honey bottles - the player will stop drinking once they get sent through the portal.
It also occurs with any other actions custom bound to a mouse button - for instance, if you bind the Move Forward action to a mouse button and using it to move through a portal in creative mode, you will stop moving the moment you go through the portal, until you release and re-press the mouse button.
Regarding impact, this bug breaks some farms, as various automatic farms operate by the player holding down left or right click (either manually or through use of the texture reload trick) while being moved about my minecarts or water streams. However, due to this bug, any such farm that transports the player across dimensions cannot be automated without use of an external auto-clicking tool, as the player would need to release and repress the relevant mouse button every time they go through the portal.