How to recreate the bug:
When the player jumps while holding "right alt"/"alt gr" and opens an inventory or a text field at the moment the player lands/ jumps again, the bug starts happening. Examples of inventories that can cause the bug include the player inventory, chest, anvil, etc, and examples of text fields include chat, signs, command blocks, etc.

Effects of bug:
1. Any time you press "a" while writing in a text field (such as command block, chat, recipe book, anvil, etc.), the whole line/text field is replaced by the letter "a".

2. Any cursor movement or pressing backspace moves/ erases until the next blank space, instead of moving/ erasing one letter at a time.

Stopping the bug:

Pressing the left "ctrl"-key stops the effects of the bug.

Video:

https://drive.google.com/file/d/1FWExzYH35RkZPegL8Wyu1a12ekDf-uOU/view?usp=sharing