In 1.21.2 when a trial spawner spawns a shulker, they can spawn on slabs, causing them to instantly teleport away

How to reproduce:
launch the game in 1.21.2
create a field of slabs surrounding a trail spawner
insert a shulker in the trial spawner by right clicking it with a shulker
activate it by switching to survival mode

Video Showcasing Trial Spawner behavior with shulker in 1.21.2

Observed result:
you will observe that sometimes shulker will attempt to spawn on the slab, which they can't and then teleport away

Do note that this isn't always the case as it can still happen that they spawn on the trial spawner itself. in which case they wont teleport away.

Expected result:
this trial spawner behavior with shulker wasn't the case in 1.21.1:

how it used to work:
launch the game in 1.21.1
create a field of slabs surrounding a trail spawner
insert a shulker in the trial spawner by right clicking it with a shulker
activate it by switching to survival mode

Video Showcasing Trial Spawner behavior with shulker in 1.21.1

Shulker will only be able to spawn on the trial spawner and stop spawning new shulker until the potential space to spawn on has been cleared / the existing shulker has been killed