When the spawnpoint is in a swamp with approximate surface height <63 and a frozen ocean nearby, cubiomes-viewer will move the spawnpoint to the frozen ocean. This doesn't happen in game. Even if approximate surface height is below Y=63 in a swamp, the actual spawnpoint will still be at or near there because the actual terrain could still be above Y=63, and even not, there's still trees where you can spawn on them. So in a swamp/mangrove swamp, when the spawnpoint has approximate surface height below Y=63, don't move the spawnpoint, just keep it there.
When the spawnpoint is in a swamp with approximate surface height <63 and a frozen ocean nearby, cubiomes-viewer will move the spawnpoint to the frozen ocean. This doesn't happen in game. Even if approximate surface height is below Y=63 in a swamp, the actual spawnpoint will still be at or near there because the actual terrain could still be above Y=63, and even not, there's still trees where you can spawn on them. So in a swamp/mangrove swamp, when the spawnpoint has approximate surface height below Y=63, don't move the spawnpoint, just keep it there.