Bubbles can b due to air in the system as well as faulty head gasket. But since u r saying that the system isn't reducing the coolant, it most probably is air in the system. Bleed air n c if it improves. Secondly, remove the cap n c if the coolant swirls in the radiator or not. If it does you most likely have a leak. Lastly remove the cap n c if the coolant splashes out violently upon pushing the accelerator pedal hard. The splashing might b subtle if it's a minor leak. You'll have to correlate the results from different tests for a final solution. I'd say, you moat likely have a minor leak. Monitor the coolant level for at least 200km. If it's reducing even 50ml, you definitely have a leak
In short for a leaking gasket, you will definitely have the following, whether pronounced or not.
1. Performance decrease
2. coolant decrease
3. Swirl in the coolant
4. Coolant splashing