Did i not give? Oh I'm sorry! (!)It would help if you gave some beneficial information. Seed name, coordinates, what settings you are using. Only way to try and replicate it.
There is an error in config file(guess) that defines playable area in v5.2 where bedrock height is changed from -57 to +3. Build-able height is still 250 (3+250=253). Affects all seeds in any configuration.
This is on unmodded version of the game- only mod is walkers via Launcher.
How to test?
* Open console
* type dm and cm
* Give yourself a nice digging tool(or uncomment >! in items file to give yourself Dev items)
* hit esc and enable god mode
* Dig till bedrock
* Feel free to repeat this as many times as you wish with any seeds.
Cheers!