Were you using a warframe with an exalted melee weapon/pseudo-exalted ability? You probably already know this but the first strike of abilities like Khora's Whipclaw, Excalibur's Exalted Blade, Baruuk's Serene Storm, Atlas' Landslide, etc. will reset the combo count on the Xoris as a special interaction so maybe it slipped your mind. Also I think detonating it mid flight counts as a heavy but you already know this probably.