Chunks sometimes render with a 1-5 second delay, before they properly render, only blocks which hold tileentities or some non-solid blocks like sea grass render properly, all other Blocks are not visible for the timeframe.
The suffocation textures and hitboxes render normally, physics and collisions work as well, even if the blocks themselves are not rendered.

When updating a block in the invisible chunk (for example, breaking a block), everything instantly gets rendered properly. (This can only be triggered with a client-sided action)

Reproducing in Vanilla: (1.16+)
1. Set a high Render Distance (this just increases the chance for this to happen)
2. Load new Chunks quickly (Creative flight/Elytra), preferably oceans (easier to spot)
3. Look for invisible Chunk parts

In Vanilla these chunks also update as soon as you enter them. The invisible Region is also normally only one Layer (only water surface or ground), this confirms that this is not a generation problem.

Reproducing using 3rd party software:
1. Send a Chunkupdate with some changed blocks
2. Wait for the blocks to render

Single block updates are not affected by this.