What I expected to happen was...:
I expected a return portal to appear, which would teleport me back to my home portal. 

What actually happened was...:
I created a portal to the nether, stepped through, and found myself in the Basalt Delta. Excited, I decided to look around the area that I appeared in, and immediately noticed that the portal was not there. I immediately hit Esc. and hit the button 'Report Bugs'. I first browsed through already reported bugs (open and resolved) and finding none that matched my bug, I began to create this report. This bug only occurred when I appeared in the Basalt Delta in 1.16.1. I had been to the nether in 1.16 in a previous world, and I encountered no problems. It's possible that this bug is tied to the Basalt Delta biome in the Nether. 

Steps to Reproduce:

Since this occurred when I appeared in the Basalt Delta in the Nether, it is quite hard to reproduce. If need be, I can provide my world seed and portal coordinates to make it easier to reproduce.  
1. Create a portal to the nether and hope you appear in the Basalt Delta.
2. Turn around to look at your portal.
3. If it's not there, you have successfully reproduced this bug.