When loading any chunk in the latest preview, they get visually and physically regenerated until they reload some seconds or minutes later. Once they properly load you will retain your position and every other stat, so your movement in these chunks does carry over.
This is deadly in worlds carried over from 1.17 or below, since world generation is completely different, you could be flying through plains and suddenly be suffocating inside a mountain, since the actual world would have a mountain there and the plains were just ghost chunks that hadn't loaded properly.

In these ghost chunks you cannot use fireworks yet they dissapear from the inventory as if you did (Which is REALLY deadly in The End).
Placing and destroying a block wont yield any drops, but instead replace itself and a block above and below with air
That behaviour is similar to MCPE-167185, yet i couldn't find any mention of the ghost chunk issue, so im filing it separately.

I believe the issue is that the game loads chunks from the seed data to ease world loading, but ends up messing up since the actual world might not be how the blank seed would generate, resulting in ghost chunks the player can freely move when they wouldn't be able to in the actual world.

Video example of the bug: 
https://1drv.ms/v/s!Aj1Iy1GB4U2cxlZQ7VySOAh-eLK7?e=i2iNyc

(Windows just wouldn't let me record, so i had to do so from my cellphone, sorry)