I'm experiencing the same issue. Googling the problem brought me here. I went almost a year without replacing my top oring. Last month after cleaning my tank I noticed my seal wasn't sitting tightly so I tossed it out and replaced it with the extra oring that came with the tank. Then about a week later the new ring had sagged inside the tank. I borrowed another oring from a friend who has the same tank and within 1 day the new oring started sagging inside aswell.
I'm gunna buy a new tank to see what happens. Did anyone have any luck figuring out the issue?