Entering (Or travelling to?) Nether-side portal turns it off

On a realm (which is connected to by players playing on Windows 10 and on the Switch) I have found that travelling through a nether portal in the nether seems to deactivate that portal, in that returning through the overworld portal creates another nether portal directly on top of or next to the old one, which is now turned off. This does not always happen, but it does happen repeatedly when travelling between the two portals frequently enough. The portal in the nether is inside a cobblestone structure, so ghast fireballs are out of the question as an explanation.

 

The world seed is -138659979. The overworld portal is at 3863, 68, 4292 and the nether portal is at 470, 72, 528. I am the realm owner and was unable to test if this bug also occurs to other players. Both portals have solid blocks covering one side so they can only be entered from one direction and the nether-side portal has a sign directly opposite it in the next block, which is the only recent change. I was not able to reproduce this bug on a singleplayer world download of the realm made during this bug report, which leads me to believe it is realm specific. Unfortunately no other players were able to help me ascertain if it is the journey to the overworld or to the nether that breaks the original portal. Because the new portals generate directly on top of or next to the old one I do not think this is a "nether portal permitted coordinates" problem.