Build v1.9.2 (the beta)
The ashen fields skill states that it gives -20% weakness to fire damage for all units (local enemy armies)
I was confused as that effect would be a buff to enemy units. I checked and found that no effect modifier was activating in battles. The battle file and screenshots are from a dragon battle, but this is present in every battle with Imrik's Ashen Fields skill. This bug is present in both the beta and the normal build, though all files and screenshots are from the beta.
I tested this with units that had a base weakness to fire (Dryads) and those that didn't (Eternal Guards). Please note that the regeneration effect shown is from the dragon and does not apply weakness to fire.
(See Screenshots "AF plus Base Weakness 2", "AF plus Base Weakness of Zero")
I also tested Ashen Fields (AF) with both KindleFlame (KF) and StarLance (SL) weakness to fire effects. Both were activated separately and simultaneously. The weakness to fire effect amount was what was expected if Ashen Fields was not active. The Blazing Lance skill was also active, but I think it didn't take any additional effect because Star Lance also was active, and the same imbued effects don't stack. (Correct me if I'm wrong on that.)
(See Screenshots "AF plus SL", "AF plus Star Lance Effect", "AF plus KindleFlame", "AF plus KindleFlame Effect", "AF plus SL plus KF", "AF plus SL plus KF Effect")
Just to be thorough, I also tested Star Lance and KindleFlame on units with no base weakness to fire. Both effects were applied correctly to the unit, but no increase or decrease resulted from Ashen Fields.
(See Screenshots "AF plus SL Base Weakness of Zero", "AF plus SL Base Weakness of Zero Effect", "AF plus KF Base Weakness of Zero", "AF plus KF Base Weakness of Zero Effect")
I also doubled check that Ashen Fields wasn't being applied to my own troops. Imrik shows a +25% weakness to fire from the regeneration ability, but no sign of Ashen Fields affecting him or any other unit.
(See Screenshot "AF Imrik Weakness Regen")
I do play with mods, but none that should affect any lord skills or Ashen Fields in particular. I have attached the battle replay, the campaign saves from both before the battle and after, and my modified.log file.