When a warden is spawned in by a sculk shrieker, it can sometimes have a chance to fail, even if there is spawnable spaces available.

A way to replicate this is to setblock a sculk shrieker with the can_summon tag set to true, then build an area like a 3x3 platform in the air, then continuously spawn the warden. Most of the time it works, but sometimes he just fails to spawn in as in the attached screenshots.

You can also do this with a 1x1 platform and get faster results.

Personally, I think the warden checks all the blocks around it to try and spawn on them, and it also checks blocks it cannot spawn on, like the shrieker, and if it picks it, it doesn't check to see any other blocks that are spawnable and so it fails.

With that, there may be a very rare chance that the warden will not emerge in the deep dark or ancient cities in a gameplay standpoint.