Rendering Issues.

I really need help, I have a world in survival mode and it was created in version 1.14.20.  I've been playing it for a while now and I realized that when I enter the Nether and come back, several parts of my map have unloaded chunks, I don't understand, and the problem only resolves if I enter the Nether again and come back, or I have to get out of the world and reload. And that doesn't just happen in one place on the map.
 
There is also a problem that sometimes I exit the Nether and the loading screen freezes, but entering the Nether is always very fast and error free. Attached screenshots, I hope you can help me.
 
 
Device Model: Motorola Moto G6 (ali)
Model Number: XT1925