Before marking as a duplicate or commenting such, please note that, while it may be related, this bug is not MC-214898. I had this occur in previous version as well, while MC-214898 is only regarding new world generation in 21w06a. It would not let me report 21w05b as an affected version as it is "outdated" (despite many people using that to be able to access old worlds). (Before saying this is invalid, this bug did happen to me in 21w06a as well.)

This bug is when you use a nether portal that is really far out in either dimension, and the chunks on the other side are already loaded due to a non-player entity (which I will be referring to as an NPE) passing through, when entering the portal, the chunks will very rarely not load except for the ones that are already loaded because of the NPE passing through. 

Steps to reproduce:

1. Go really far out in either the nether or the overworld. (This increases the chances of this occurring because your game will already be lagging a lot.)
2. Place a nether portal.
3. Enter the nether portal. This way there is a portal in the overworld as well.
4. Wait a few minutes for the chunks in the nether to unload. (I am unsure of whether this is necessary.)
5. Send an NPE through the portal. This ensures that the chunks surrounding the portal will load, so that the nether is loaded.
6. Enter the portal.
7. There is a very small chance that the world decides it will not load any chunks period. It won't even try regardless of how long you wait.
8. Reload the world. You won't be able to pass back through the nether portal.

If it doesn't work on the first time, you may need to try again. You can use the same portal over and over and it just suddenly glitch out like this, as well.