when a block is placed next to an end portal block with an eye of ender in it, you can see through it

I was placing blocks around my end portal and noticed that when I place a block next to one of them that side turns transparant

what I expected to happen:It woud keep it's texture on all sides

what actually happened: the side of the eye facing the block is transparent

steps to reproduce:
1. activate an end portal
2. place a block on the side of an end portal frame
3. the side of the eye facing the block should turn transparent