The Wildfire feat is supposed to spread a max of four stacks when a target with stacks of soulblaze dies to nearby enemies. However, using a force sword with the Bloodthirsty IV + Blazing Spirit IV demonstrates that not all four stacks are spreading.
Using the force sword shown below, you can easily set one of the elite gunners on fire with exactly four stacks, which deal 23 damage tick. When I kill that burning enemy, nearby enemies only burn for 6 damage a tick. Since that weapon applies four stacks and Wildfire spreads a max of four stacks, the damage-per-tick should be identical.
This bug has a 100% reproduction rate. Screenshots are taken in the Steam version of the game. I have closed and re-opened the game between tests, and the results are always identical. Enemies with 4 stacks ticking for 23 damage die and nearby enemies only burn for 6 damage a tick.
Sword used for testing.
Four stacks of soulblaze from Blazing Spirit.
Damage of soulblaze applied by wildfire when a target with four stacks from Blazing Spirit die.