As of the 0.1.5 update, players have noticed that their Max Health has reset, on it's own this wouldn't be more then a simple inconvenience. but I've done it seems that Max Health resets on death or when rejoining a server, like how set-stat Speed would work normally. It isn't just a UI bug, since after The health reset, a Sprigull still did 2 mini-bars (or .5) damage, and the de-saturation effect still occurred, I tested by changing my health via set-stat, but it seems that additional Max Health gained from Heart Fruit (flowers? bulbs?? whatever) resets in the same way.

Edit: Just after making this post, it looks like someone has discovered that if you find another Heart-whatever after your Health resets, it'd return your lost health in addition to the extra health that the Heart plant would normally grant.