It takes a couple of seconds for the bike to be hot enough to burn the oil. I remember than from back when I overfilled mine. My friend has an old TT600 and it does then same.
Enginebraking from cold, and /or from high rpm's should show smoke. Normal enginebrakeing (like from 3500rpm wont always show smoke on a hot bike.) But i've seen than happen with both bad piston/rings, and bad valveseals.
Its oil for sure.
You could do a leakdown test.
Ride it for the summer, keep an eye on the oil, and rebuild the bike when the weather turns bad.
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