Hi Nabeel,
Certainly not the usual pattern, but very possible.
The long head of biceps often ruptures after the Supra has given way as it is overloaded in it’s function as a humeral depressor. But the biceps can certainly rupture in the setting of an intact supra. Remember the long head of biceps is so badly engineered with a curved course and soft tissue origin.
The subscap can also be torn with forced external shoulder rotation. This is common with falling injuries where the person tries to grab and hold an object to break their fall. The CHL does not necessarily need to be damaged. It does provide stabilisation for the long head of biceps and is intimate with the subscapularis, however it may survive in the setting you describe.
The usual pattern is for the supra to tear, the CHL is weakened, the long head of biceps migrates medially , the long head of biceps is overloaded and ruptures.
But anything is possible.
When you think the subscap is torn it is worth trying a “lift off test” . Put the back of your hand on the small of your back and try moving the hand away from the spine. Test the strength on both sides and look for a reduction on the torn subspace side.
I think what you observed is just fine. Unusual , but fine.
Steve