Beacon effect issue

We use a beacon on the overworld origin spawn point.
The coordinates of the beacon are : 69 / 68 / 254
Our beacon are a full structure so a range of 50 blocs.

The coordinates of the nether portail of one of your users are : 26 / 53 / 307.

When he goes the path "Overworld -> Nether" and at these coordinates, he gets the beacon effect. When he goes in the nether to 18 / 53 / 307, he loses the beacon effects.