I experienced a boss fight on Stage 17 of spirit keep, in which my heal reduction champion Lifetaker (lvl 50) was not being revived despite being down for a good 5-6 minutes, and numerous Godseeker (lvl 60) turns.  I watched the fight from around minute 6 to the start of minute 12.  At minute 12 plus, the battle completed, but unfortunately I did not see how it completed, though my natural conclusion is that Godseeker revived Lifetaker, her healing reduction landed, and the boss was defeated.  Other things that might be of interest: that the 4 champs that were alive were in no way low on health (>90 or 80 % eyeballing it) and the team managed to eat through 50% or more of the bosses' health before she took a turn and healed up.  

P.S.  This has been the only time that I observed this behavior despite many spirit keep runs at various stages.