When playing survival single player, I always, at some point in the game, see random unloaded sections of the map.

Here is a list of some observations :

• Location and time seem to be random
• It can be either on a chunk not generated or an existing one
• It often is a couple of them side-by-side.
• Changing the orientation of the character doesn't affect it.
• Approaching the missing chunks make them suddenly and consistently appear.
• The distance at which it appears may vary, but it always is in the range of 1~5 blocks.
• Changing a video settings (ex: draw distance) seems to solve the issue by forcing the chunk to render again.
• When playing creative and flying, it seems to be more pronounced.

I attached a screenshot of a fresh new game, where you can see 2 sections of the map missing.

Side note (may be related): I observed that my maps are generating very slowly (this is a consistent behavior), usually between 30s a 1m for a render distance of 14 chunks, and this only for a single angle of view.