Terrain generation boundaries

When opening a pruned world originally generated in 1.12.2 there are some interesting chunk boundaries in Mesa biomes. As can be seen in the screenshots, on both sides of the boundary where the chunks were kept/deleted it previously showed the biomes as "Mesa Plateu F", now on the kept side it's "mesa_rock" and on the newly generated side "mutated_mesa", which results in a clearly visible boundary.

I do not know if this is intended/expected behaviour or a bug, as I could not find any information on this. My understanding was that changes in terrain generation should not create chunk boundaries this time?