Region File Corruption when downgrading from 26.3

I updated my Singleplayer world from 26.2 to 26.3 but due to using a modded client I wanted to switch back.

When I joined my world after downgrading back to 26.2 the terrain below me cease to exist and I fell into the void.

After enabling cheats and flying near a base the terrain also got removed as if it never existed but I could see entities falling into the void and I got death messages from a few of my cats. Luckily i had a older version of the save backed up.

I have done some testing using completely vanilla versions of 26.2 and 26.3 and upgraded a world from 26.2 and generated a world in 26.3 which i then both downgraded back to 26.2.

All Chunks seem to disappear when downgrading and loading them in, new chunks do still generate successfully.

I checked the regions files on one of my test-worlds and found that one contained the expected data the windows editor is unable to decipher correctly and other files looked way different after taking quite a while to be opened.

I believe when downgrading the region-Files get massively corrupted

Attached is a screenshot comparing 2 of the mentioned region-Files, I believe the upper right one to be corrupted and the lower left one to be healthy. I have also provided screenshots of one of my test worlds generated in 26.3 (the normal snowy terrain) and after the downgrade where you can also see the normal chunks generating again. before downgrading the world I didn't leave the world spawn and only flew up a bit to get a better view. I also have included a screenshot where you will be able to see some pigs fall into the void after the terrain disappeared.

After further Testing i can confirm that the same thing appears when downgrading from 26.3 to 26.1 so the issue seems to be with 26.3. which also brings the terrain-blending on bordering chinks to its knees.