Blocks like glass, rails, nether portals, leaves, and ice are loading in the sky, but still "show" in their correct places whilst loading into 1.17 from a previous 1.16.4 world, screenshots are attached. (this also happens in newly generated worlds.) This is causing my game to lag even with the new release of Optifine.

NOTE; i did use MCA Selector to delete chunks that i didnt need, but left a reasonable amount of space between generated chunks to ensure that nothing was deleted, however i dont feel like that is the reason why the nether portals glass & what not are bugged like this.

After further investigation this does happen on every other world that I load in so it doesnt seem to be a problem on my end using applications to delete chunks.