This world was created in 1.16.2. And I have not modified the game with mods. I only have a resource pack that allows me to see the chunk borders. 

The problem is: 

I had one nether portal (1) and it was working fine. But as soon as I created a second one (2) (a couple 100 blocks from each other in the overworld) the problems began. When I entered portal 2 in the overworld it sent me to the location of the portal 1 in the nether. And there I could not come back, after trying for 20 minutes everything that came to mind it suddenly worked and I was transported to the location of portal 1 in the overworld. 

I tried manually making the portals in the correct location in the nether first, before trying to use them (dividing the coordinates by 8 and using online coordinate calculators).  It seemed to work well for those 2 portals.

But now I needed to build 3 more, that are some 20 blocks apart in the nether. And those only work in one direction. To the nether.  And I can't use any portal to come back. For some reason, can use a portal that is like 1000 blocks away from those 3 and the portal 2 (the one I talked at the beginning) in the overworld.