The Ring of Temporal Salvation says (emphasis mine):
If you die while wearing this gray crystal ring, you vanish and reappear in an unoccupied space within 5 feet of the space you left (or the nearest unoccupied space). You have a number of hit points equal to 3d6 + your Constitution modifier. If your hit point maximum is lower than the number of hit points you regain, your hit point maximum rises to a similar amount. If you have any levels of exhaustion, reduce your level of exhaustion by 1. Once the ring is used, it turns to dust and is destroyed.
If used by a 1st level character, 3d6+CON can easily total more than your maximum hit points. Is this increase to hit point maximum permanent?