Can't fly through an opened Shulker Box in Spectator Mode

Steps to Reproduce:

1. 2 players needed, one of them start a world.

2. Then ask the other to open a shulker box, you change yourself into spectator.

3. Try to fly down on the opened box.

Observed Result:
You'll find you sometimes can't fly down through the box.

Expected Result:
You should be able to fly down easily through that.

See attached video.