Left: A horizontal stair (facing: west, half: top, shape: straight)
Right: A vertical stair (facing: west, type: vertical)

When a texture is applied to the horizontal stair, part of the vertical stair becomes invisible:

Same thing happens on the mirrored version btw:

forge-1.16.5-36.2.34.jar
FramedBlocks-2.15.1.jar
No Optifine

Nice catch. Fixed in dev, next release is still some time away though.