When you use /fillbiome the biome will only be set in patches that extend outside of the range you set and in-between the patches the original biome will still remain. Also the command will only set it on the y axis you included in the range (may be intentional). Also the command causes some visual glitches.
Idea:
Remove the Y axis from the command's range.
This is unfortunate as this command is so useful.
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Here's a link to the changelog for future reference: https://www.minecraft.net/article/minecraft-snapshot-22w46a
See also MC-160939 – it was about this change in how biomes work.
The visual issue is MC-257648.
This is intended, since 1.15 biomes ave been saved per ~ 4x4x4 blocks, rather than 1x1,to support cave biomes and alike. The changelog also states this explicitly.
As for the visual glitches, please search if they got reported already, and if not, make separate reports for them (1 issue per ticket).