4 things suddenly came to mind.
1 As many have written before me, it can make you cough when used together with traditional cigarettes or even during the few weeks of acclimatizing.
2 Too hot vapor, too high wattage for the coil, poorly calibrated TC mode.
3 Some kind of allergy, possibly nickel, in which case SS316L is not good because it contains it too. What is nickel-free is Kanthal or SS430 (it’s worth ordering from a reliable source, no-name Chinese brands will print whatever the customer asks for…).
Or it could be PG intolerance/allergy, or the body is reacting to something in the flavor ingredients. I’ve also encountered flavors that made me choke. Have you tried multiple liquids? Different manufacturers, different flavors, maybe a thinner mix?
4 Possibly nicotine? If you’ve tried freebase, it might be worth trying nic salt too.
Unfortunately, this is a trial-and-error process, and it can be quite difficult to figure out what might be causing it.