Elytra can't fly trough 1 block high gab.

while testing the swimming mechanics and testing elytra. i found i could not fly trough 1 block high gab. i would enter a little bit and then be pushed out and fall to my death. i tested it also in creative and found i go in 1 block high gab if it was a slab or glass on top, but when i tried with full blocks i could not enter.