devtrackers.gg
I don’t think there’s a bug here, just a bit of misunderstanding (and lack of clarity on our part). If you watch your character sheet until you run out of Ward (shown above the health globe), your stats should return to normal, as they did when I tested.
The amount of protection you have is the second number under each damage type in the character sheet. Protections increase how much effective health you have against that damage type, which results in you taking a percentage less damage. This percentage is automatically calculated for you in the character sheet.
When you gain Ward (from Flame Ward or otherwise), your total health pool (health + ward) increases, but the amount of protections you have stays the same. This means the percentage goes down, because you have more health.
We do plan to explain this better, but hopefully this helps!