How often it occurs: Pretty Often. More present with faster mining tools. Is repeatable with any tool. Likelihood of it occurring reduces when mining speed is closer to vanilla values.
What should happen: Should be able to dig straight down.
What happens instead: You should not be having your head snapped above you when your trying to break a block under you. You are forcibly made to look in a different direction, normally straight up. I was not making my player look up after breaking a block below me.
In the 2nd videos case, sometimes the block refuses to break under you, and you have to try multiple times to break it or get off the block entirely.
Recorded Video:
Another example provided with the help of a fellow pirate:
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