Studied and also suspected something must be wrong with the fuel injectors. A partially clogged or fully open/closed injector can cause this.
Anyways, did the TB cleaning this morning, and poured very little carb cleaner inside the fuel rail. It still emitted black smoke / unburnt petrol smell from exhaust. But after driving to office, I didn't find any such issue.
Will observe tomorrow morning, and if issue persists, will proceed for injectors cleaning (hopefully no further steps would be required
)