In 1.18 the biomes are laid out in a 5d grid of noise values. Usually all grid cells are filed by one biome. However, if a grid cell is not filled, the world generation will pick the biome from the cell with the closest values.
For the combination of peak weirdness, coast continentalness, and erosions 0 to 3 there are no biomes defined. This causes the generation to sometimes pick ocean biomes (that are in the cells with lower continentalness) leading to them generating on land. These are the only grid cells that have no biome definitions.
Here are some areas where oceans generate on land (sometimes very high up in the mountains) because of this bug. All of these positions are on seed 1 . (updated to 21w41a)
Ocean on land:
Ocean on peak: