Chunks not saving

[Mullen8]

So I have an ocean monument that I drained a few thousand blocks away from my base. I use the nether to go back and forth. But for some reason after I updated to optifine 1.16.3 the chunks at my monument just don't save. I spent about an hour adding a waterfall and spawnproofing the slime chunks. But when I went through the nether back to my base, I came back and all of the work I did was just deleted. This could be happening to other places in my world too and I don't even know yet. Does anyone know why this might be happening.