I don't think that this biome should have this name, when a much better description existed before 1.13 (Sunflower Plains)
MC-131441
Sunflower Plains are called mutated plains
Resolved
Invalid
0
The biome is still called "Sunflower Plains", but the biome's ID name is "minecraft:mutated_plains", which is shown in the debug screen as you've already seen. This is should be a bug with inconsistent IDs, which means this ticket can also be said as a duplicate of "MC-123540 - Some ids are unintuitive".