This issue exists since patch 49.2 and the changes to Orphea.
The new talent
Chaotic Assault (Lv.4) is meant to increase the damage of your next basic attack when at 3 stacks of chaos.
Abyssal Symbiosis (Lv.13) instantly grants 3 stacks of chaos when hitting a hero with chomp.
When 3 stacks are gained from chomp in this way, Chaotic Assaults bonus damage is not granted to the next basic attack.
This does not occur if an additional ability (or another cast of Chomp) also hits an enemy hero before the basic attack is fired. It does not matter if this second ability is cast before or after 3 stacks of chaos have already been obtained from Abyssal Symbiosis + Chomp.
Tested in practice range at level 20, with both talents selected:
Chomp → 3 stacks gained → AS does not trigger.
Chomp → 3 stacks gained → Shadow Waltz → Works.
Chomp → 3 stacks gained → Chomp → Works.
Shadow Waltz → Chomp → 3 stacks gained → Works.
Shadow Waltz → Dread → 3 stacks gained → Chomp → Works.
This bug was initially reported here by another user: