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.
Thank you for your report!
However, this report is Invalid.
Your game is modified with Optifine. Optifine is a very heavy game modification, and using it will automatically invalidate reports on the bug tracker.
If you can reproduce the issue in a vanilla game environment, please create a new ticket - this is a bug tracker for the game, and not a support website.
Any issues occurring from using Optifine need to be taken up with the developers of Optifine here, not Mojang Studios or the Bugtracker.
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