Many features are not 100% accurate, i.e., locations can be wrong or missing. Near coasts, ocean and river biomes often have land, while land biomes can be underwater.

This is not a bug, but because the map shows the underlying biomes instead of the exact terrain. The coastlines for versions 1.18 and above don't match up perfectly. Always select the version that was used to generate the chunks you're looking at. If you started your world on an older version, you'll have to switch between the versions in the app.

(for slime chunks) you didn't wait long enough, since they have a very low spawn rate even in slime chunks.

you used mods or resource packs that affect world generation.

you selected the wrong edition or version (the one used to generate the region you look at).

you entered the wrong seed or coordinates.

If your in-game world doesn't match the map here at all, it's most likely due to one of the following: If none of the below helps, please report in the comments and include your seed, platform and version, and coordinates.

Minecraft 1.12 maps city