Apparently the rna_Armature_editbone_transform_update function was incomplete because it didn't copy all mirrored transform values.
I tested it in Blender 2.79 and the problem is also seen there.
I don't know if it ever has worked properly.

I also noticed that the rna_Armature_editbone_transform_update logic is also seen in ED_armature_edit_transform_mirror_update.
So the solution proposed in the patch is to expose and use the logic that updates a mirrored bone. Thus deduplicating and fixing T65671.

It seems that the original code was written here: rBf1a745c43688