If the shulker is on a team with the collisionRules set to never, it still has a collision.

How to reproduce:

• Add a team: /scoreboard teams add test
• Change the collision rule: /scoreboard teams option test collisionRule never (or pushOtherTeams)
• Summon a shulker in the test team: /summon Shulker ~ ~1 ~ {Team:"test",NoAI:1b}

  or let a shulker join the 'test' team (/scoreboard teams join test @e[type=Shulker]).

• Try to stand on it, you will not pass trough the Shulker.