That’s what I just said. The hit point maximum reducing effect of the bite is not normal. It reduces your hit point maximum. After that, normal processes resume, and your hit point maximum stays at its new reduced value (until you finish a long rest).No it's not. what happens normally is your hit point maximum is unaffected entirely.
The vampire's bite is what reduced your hit point maximum. The reduction is just what it is, the fact of your hit point maximum being less than it was before you were bitten. That's what continues to be the case until some hit point maximum restoring effect undoes the effect of the vampire's bite. The reduction caused by the vampire's bite has the special property that it can be undone by finishing a long rest. Your hit point maximum staying reduced until an effect undoes the reduction is perfectly normal. Normally, your hit point maximum doesn't change from whatever it is unless you gain a level or increase your CON modifier.There is no "normally" when talking about maximum hit points being reduced. Something reduced them, and something is keeping them reduced or there would not be continued reduction.
Your hit point maximum is reduced because that's one of the effects of being bitten by a vampire. The bite does what it says it does.Why is max hit points reduced in the first place? It's not necrotic damage that does it, because lots of things do necrotic damage and max hit points don't get reduced.