Weird...when I tried it now...he dug down normally...maybe one time error.
Confirmed in v.20w16a
In 20w06a it it solved. But there are some"ice spikes" everywhere and there is an water cube with ocean monument.
In version 1.13.2 and older.
Ïn older versions it generates normal world with an river near spawnpoint.
Again in another world
Yes he was...I was afking right next to him...