As of reporting this bug it has happened 3 times in the last 15 minutes. When I log in and out of the world it fixes it.
What happened was I walked in chunks that I've already loaded and when I get to this part (see image) I fell thru!
The bug was triggered by loading in chunks (old and new).
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duplicates 1
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Thank you for your report!
We're tracking this issue in MC-227996, so this ticket is being resolved and linked as a duplicate.
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Please take another screenshot of it happening with the [F3] debug menu enabled, then attach it to this ticket.