Ok - that’s slightly more odd.
Look. Best thing you can do is install 4.25/7 whatever, do it there, then migrate that project to 5.
It will save you from chasing your tail to figure out why by either not working - at which point you know its an engine bug and you can report it - or by working, so you can analyze it and figure out what to actually do in the new engine to make more holes.
Otherwise stuff to look at is probably landscape layers (multiple landscapes modifying each other, one without the hole?), or the default setup for the landscape…