What happens when a wildshaped druid that has had its HP max reduced reverts back to normal?

@JeremyECrawford What happens when a wildshaped druid that has had its HP max reduced reverts back to normal? Does the reduction carry over?

— Jonathan Longstaff (@pukunui81) May 26, 2016

Wild Shape—a reduction to hp maximum doesn't carry over from your beast form to your true form or vice versa. #DnD https://t.co/S312Jp59N5

— Jeremy Crawford (@JeremyECrawford) June 4, 2016

@JeremyECrawford Thanks! Follow-up: If vamp spawn drains druid's beast form hp to 0, does druid die or revert to normal form?

— Jonathan Longstaff (@pukunui81) May 27, 2016

Wild Shape—you die if a vampire spawn reduces your hp maximum to 0 while you're in beast form. #DnD https://t.co/GJRfbjP65d

— Jeremy Crawford (@JeremyECrawford) June 4, 2016

@JeremyECrawford @pukunui81 Only the beast hp maximum or druid form too?

— Manny (@techjunkie30) June 4, 2016

Whatever form you're in, you die if a vampire spawn, wraith, or the like reduces that form's hp maximum to 0. #DnD https://t.co/YB8rTyflQT

— Jeremy Crawford (@JeremyECrawford) June 4, 2016

@pukunui81I don’t suppose you could clarify *why* this is the case (and not the reverse, as you initially said)? The forms have their own hp pools. Their current and max hp change independently of each other.

— Jeremy Crawford (@JeremyECrawford) June 8, 2016

@pukunui81Also, if current and max hp change independently, why does excess damage carry over? Just want to understand the thinking. One hp range absorbs as much of the damage as it can, and the other hp range picks up the slack.

— Jeremy Crawford (@JeremyECrawford) June 9, 2016