Whenever you light a nether portal or place slime/honey block the texture disappear and appear in the sky, this bug occurs in overworld/nether/end, but when a slime or honey block is placed on a piston/sticky piston the texture appears for half second,

and also the texture of the water glitches , between every falling water block a thin block (like a carpet) but have the color of water but darker,

these bugs occur in a flat world with redstone machines with slime/honey blocks,

and it can also happen when you update your 1.16.5 world to 1.17

(About The Photo):

The Nether Portal Is Actually Opened!