I haven't noticed this happen on repeaters though, so I assume it has to do with the redstone not having a collision box?

I've also noticed them spawning in 1 high places where they wouldn't be able to open their shell, which I assume shouldn't happen given this quote on the shulker's wiki page

However, if the block the shulker occupies becomes something other than air or the shulker finds itself not adjacent to any solid block, or the shell cannot open, it attempts to teleport

Shulkers and a few other mobs are meant to spawn in different conditions than other mobs - I haven't come up with it - its Mojang.