While writing in book & quill there is a chance for ctrl to get stuck and cause entire text to be deleted when pressing a. Pressing ctrl+a fixes the bug.
While I am not completly sure, the bug could be caused by polish letters ą, ę, ł, ć, ż, ź as the bug seemed to come up most often after writing words heavy in these letters such as "wyłączyć".
Related issues
duplicates
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violine1101
Thank you for your report!
We're actually already tracking this issue in MC-127862, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.