Monsters often brush under or outside platforms and fall directly into the void.
Looks like a Carpet problem.
There are still ways to build spawns, but I think you should know this.

Monsters often spawn in the void and fall to their death.

Do you mean they spawn in the entire bastion structure even without a platform and then directly fall into the void? That's interesting to know.

Monsters often spawn in the void and fall to their death.

Do you mean they spawn in the entire bastion structure even without a platform and then directly fall into the void? That's interesting to know.

No，There is a platform nearby, spawns will not spawn on the platform after spawning

This is due to the fact that Minecraft allows Piglin Brutes are allowed to spawn in mid air. I assume Mojang never gave them spawn restrictions because they're only supposed to spawn on world gen. Will release a fix shortly.

I opened an issue (gnembon/fabric-carpet#1335) for the Carpet mod but this should be fixed in 3.1.1.

Monsters often spawn in the void and fall to their death.