On Mythic Cenarius, Nightmare Ancient is defeated twice, but WCL shows only the first Nightmare Ancient.
The first Nightmare Ancient is killed when it appears (00:25), and it shows properly on the chart. The second one is cleansed (02:48) when it appears, and then corrupted about 1 minute afterward (04:00). WCL categorized the damage dealt on corrupted Ancient as damage on ally.
I am not sure that the issue is about WCL or combatlog itself… I am glad you to look into the issue. Thank you!!