Copycat slopes render incorrectly when viewed from certain directions, this issue seems directional so it might not show for all placements.

To recreate place the slopes and view them from above if the angled side is down and so on. This also goes for sideways blocks.

I suspect that the blocks are just culled out because its handled as a straight blockface facing down instead of an angled one facing both down and sideways.

I don't have the time write a well formatted report sorry.