It is technically ambiguous, but the intent is clear: Your hit point maximum returns to normal when the disease ends
The sentence structure can be parsed in two ways:
[Until A], [B and C]
[Until A, B] and [C]
This is technically ambiguous, though I would assume the first is the intended reading as this simply makes the most sense. Having the maximum HP reduction sit around permanently is extremely punishing. And from a CR 1/8 creature no less.
Do note, however, that in neither case is it actually permanent. There are effects that would specifically end the HP reduction even if it were somehow intended to never end naturally. For example, greater restoration:
[...] end one of the following effects on the target: [...]
- One effect reducing the target's hit points maximum [...]
Other similar features are not permanent
The Specter's Life Drain ability states:
[...] The target must succeed on a DC 10 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the creature finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
This is a CR 1 creature, and I would not expect the CR 1/8 rat to have a significantly stronger effect that can only be ended by a 5th level spell. Something no party fighting a CR 1/8 creature would have meaningful access to. While, technically, its effect is more specific, in that it explicitly details how long the reduction lasts; I would say the rat's feature does as well, it lasts until the disease is cured.
Compare this to the fact that, a party fighting a CR 1/8 rat is far more likely to have access to methods of removing diseases, for example: the 2nd level lesser restoration spell and the Paladin's Lay on Hands feature.