3x3 nether portals unlight in the overworld if you log out of the world or go far away in the overworld or nether. From what I’ve seen it only happens with 3x3 portals and all of the regular 2x3 portals have never unlit for no reason. If it matters the portal in the overworld is 3x3 but the portal it connects to in the nether is 2x3. Also interestingly, if the portal unlights in the overworld but you are in the nether, the nether portal in the nether stays lit, but if you go through it to the overworld it generates a new nether portal.