There is a new bug in Minecraft 1.16.3 and it happens when the player has open their inventory either when they're on an entity or not. When shift button is pressed while the inventory is open, the player sneaks too, making them to exit from minecarts, horses etc while having inventory open. Fix that bug please.
The bug occurs on the default/unmodified vanilla client to modified clients like Forge or Optifine.
Please provide a screenshot of this occurring with the F3 debug screen enabled.