When working with the new Mojang mappings and the specialsource-maven-plugin I came across some issues.

Some methods are not mapped correctly to the Spigot mappings.

var listTag = new net.minecraft.nbt.ListTag();

//These methods work fine
listTag.add(net.minecraft.nbt.IntTag.valueOf(10));
listTag.addTag(0, net.minecraft.nbt.IntTag.valueOf(11));

//Missing method
listTag.add(0, net.minecraft.nbt.IntTag.valueOf(12)); 

//Should return 10 but method cannot be found
getLogger().info(() -> listTag.get(0).toString()); 

For example the net.minecraft.nbt.ListTag.add(int, Tag)

The method as in the Spigot-mappings:

net.minecraft.nbt.NBTTagList.add(int, net.minecraft.nbt.NBTBase)

Actual remapped method using maven plugin:

net.minecraft.nbt.NBTTagList.c(int, net.minecraft.nbt.NBTBase)

Which of course throws a NoSuchMethodError https://pastebin.com/WGjZT5Vz

Maybe it's because they have the same name in the Mojang and Spigot mappings?
I could imagine there are more mappings affected by this.

I've made a simple test plugin with just the code above and maven.