I may be talking out of my azz here, but if the XXX8 series bikes are similar to the XXX9 engines, and you were to put a wide band o2 sensor in each midpipe for sampleing and tuning purposes, you will find that one head and pipe will run a little hotter and leaner than the other one.
Are you running your factory pipes still? If so, it could be condesation in the longer pipe to the front head taking longer to burn off.
If not, what pipe/ muffler are you running?
Ever had it tuned or remapped to match the pipe/muffler combo?
Did you only start noticing it after the service work, or perhaps your more in tuned to watching for oddities now after the service than before.
If your sure you did not have this issue before the rockers were changed, Im wondering if some slight dammage occured to one or more of those valve seals.
Did you have the work done at a dealer or a certified Ducati service center?
Its possible the technician was not familiar with our sensitive valvetrain.
Im just trying to think of possibilities and questions to help.
+1
You can easily have a first indication: Is it white, blue-ish or blue smoke?
Basically white is condensation, and should go away upon heating up. It is more visable during colder and humid weather.
Blue-ish might be a start of a problem, or, if you are lucky, the result of left over oil-films after a engine job. Then it will burn off and disappear after heating up one time.
Blue smoke normally indicates oil leaking, and is an indication on a problem like pistons, valve seals, etc.
It might also indicate a repair job gone wrong.
This will not go away upon heating up (although it might get less) and is generally visable upon blipping the engine.
Advise you to give the bike a good ride (>50km) heat up everything properly and see again after it has cooled down.