Some "Floating Island" worlds are created as default worlds (not islands). This includes gravelly mountains, gravelly mountains+, and badlands plateau. This last one, you inevitably experience the additional indignity of spawning in solid stone at 8.5 65.0 8.5 (see MC-190724).
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This appears fixed in 21w37a though the terrain is messed up due to MC-236723
I've removed your point about the ocean & river floating islands worlds, which is WAI as per MC-190083.