Doing the Nightfall last night with stabbim when I noticed something weird going on with the Firebolt grenade. Only happened to Knights, as far as I could notice, though.

Are you using Touch of Flame and Viking Funeral? From latest patch notes:

Passives
* Viking Funeral: no longer extends the duration of Ignite effects
* Viking Funeral: now weakens targets afflicted with Ignite effects, causing them to take 5% more damage from all sources, stacking up to 3 times.

First hit would ignite them, following burn they would take extra damage (I assume). If it's like Sunbreaker melee or Shadowshot it will cause the numbers to turn yellow.