Glass blocks next to water will partially de-load at certain distances away. It is most noticeable when flying with an Elytra. It doesn't matter if you are in the chunk, the glass will look strange and pixelated, more than usual. I've tested this in Optifine and in the Vanilla game, water and glass don't look right when this bug occurs. It happens fairly often. The bug will also occur sometimes with regular ice blocks. I've attached screenshots from my Hardcore world, and a screenshot of the bug occurring in one of the Popular Youtuber's video, Mumbo Jumbo. The texture pack nor render distance affects it.