Bleeding can't explain dying with big top after revival. First of all, bleed can't kill you. Second of all, he'd need to have 10 stacks of bleed and 5 seconds of it to get to 1 HP from it before Big Top's invincibility expires. Third of all, Sax only has one bleed move, and it's a single hit SM, so it's not even possible for an Epic Sax to have given that many stacks of bleed. And finally, Strike Up The Band doesn't inflict bleed.
The only ways that I can think of for him to die during strike up the band that would not have been caused by a bug would be if there was a peashooter on the opponent's team giving his Strike Up The Band armor break properties to remove the invincibility status upon revival, or if he was playing the Valentine PF and his Big Top had a status inflicted on her when Epic Sax used strike up the band, so the modifier made one of the hits inflict armor break after he revived.