Affects 1.16.2, it appears that the game does not check the target block to teleport to is a solid block in the deobfuscated code. (net.minecraft.world.entity.monster.Shulker.teleportSomewhere())
Can confirm in 20w30a.
Couldn't reproduce in 20w30a, guardians spawned fine for me.
Confirmed in 1.16.1
Affects at least as far back as 1.8, with the following configuration.
[media]