Observation: The block fails to get rotated in:
GeometryUtils.rotateCoordinatesAroundYAxis(..) ->
BlockUtils.rotateBlockStateWithFacingProperty(..)

Solution:
Both the class StairsBlock and BlockStateProperties defined a FACING property for blocks.
The FACING propery StairsBlock is a redefinition of BlockStateProperties.HORIZONTAL_FACING.
Solution was to add support for rotation of blocks with the HORIZONTAL_FACING property in the rotateBlockStateWithFacingProperty() method.

Closed with commit be1918d.