Fire Damage over time starts ar 10%, and increases to 80% at Rank 37.
At Ranks 38 and above, this goes to 0%.
Update: With a different set of gloves, I only hit 78% at Rank37, which seems a bit odd.
Since it’s possible that gear combinations may get you to 80% at different Ranks, it may be that the bug isn’t directly tied to a specific Rank of Fireball, but a specific burning damage amount or percentage.
Thought I’d add that if anyone else is seeing slightly different values.