You're right it can't go in waste slot
It's not a bug but it still needs to be fixed because otherwise the mechanic of keeping eggs inside so they don't rot becomes strictly worse
Because even if it has little chance of happening with enough time and enough machines it will happen eventually.

I did more testing even when the slot exists it cannot insert spoilage into captive biter spawner

a inserter can't insert spoilage If the waste slot does not "exist" (And it doesn't make the waste slot apear)
it can put it in the egg exit slot If it's not already full of them
This is a problem because the bioflux can rot inside the inserter (inserter don't try to put spoilage in when there's no ...

a inserter can't insert spoilage If the slot does not "exist" (And it doesn't make the slot apear)
it can put it in the egg exit slot If it's not already full of them

(I'm going to make a new post for it)

I recently noticed that a large part of my Captive Spawners no longer had a waste/spoilage slot
Considering the patterns of those who still have it and those who no longer have it, (The older ones still have it, The new ones don't) it is probably broken by an update
I tested with the map editor and ...

If I'm in remote view and another player pauses the game I can't open the Escape menu (And I can't do anything else)
I'm the host even if I don't know if it's important