islands generating a flatworld instead?

[TheGlitteringOne]

I have been using islands for a while now. Have 11 detailed islands of various types in the world. When I fly out past the furthest island- the world seems to now be generating a 1- 4 block thick flat world starting outside of the 4th island outwards from center.
The island in the picture is the last generated. The flat appears as you move instead of open space. As you can see in pic 1- I quickly climbed away as it was generating them- you can see both the lead and trail edges... This has never happened before- I have flown way out for video etc as islands have been added and worked on over the span of a year.

My questions are how to stop and undo this? Remove the flat garbage? Can this be fixed?

All settings checked and cleanroom and voidgen work fine- server and plugins up to date-1.18.2 paper- workaround has been to set world border and painfully painting it all air/ x y set 0.