I'm actually not sure I'd classify the firing flamer on the dead mech as a bug - battlefield hazards from ammuniton and weapons on downed mechs is neat.
However, it should probably do damage when you walk through it. This one showed the damage indicator around the edges of the HUD, but didn't affect my paper doll.
As shown in the video, this is on the Forest Colony map up towards the elevated enemy base.
I haven't been able to reproduce this nor seen it again (I guess mechs with their flamers firing when they get killed aren't very common) so I haven't been able to test whether the lack of damage is a fluke.
(I know, I know, probably the flamer still firing on a dead mech is unintentional, but I like it!

Video taken on Nov. 1st at 21:51:58 PDT (GMT-7).