Encountering this issue 100% of the time.

Minecraft dedicated server (Bedrock).

I built a nether portal to explore the nether. This works fine. My friends joined the server, and built their own Nether portal. To their surprise, they ALWAYS came into the Nether through the same portal we came in with.

To our further surprise, when they came out of the Nether through the portal they came in with another time, they came out of a NEW portal that was generated about 100 blocks from the original portal that I had created. There are now 2 nether portals near my original location, and when the one the server generated gets destroyed / disabled, the server ALWAYS generates an additional portal as close as it can to the undesired portal location.

In short: 

We have 2 user created Nether portals ~300-350 blocks apart. They both link to the same portal in the nether. When the portal in the underworld is used, it ALWAYS sends us back to a NEW portal the server creates, about 100m from the first player created portal. Attempts to destroy the server created portal result it being regenerated when a user comes back from the underworld.

Attached are:
2 user created portals (1 unlit, one with snow on it)

1 Server generated portal. Observe  the obsidian from the portals we destroyed when trying to work around this bug.

Link to .zip backup of world: https://1drv.ms/u/s!AvzUjoCiXaakmJsVHm-QVYBEFDYRSQ?e=0c0ePe