It's the most reasonbale hypothesis, such as compulsive rubbing of the lips while using products!
I still think that eliminating the inflammation is the main point of resolving EC. Because of repeated trauma and rubbing, it acts like psoriasis, without being it.
About the wetting of the lips before applying, it's kinda difficult inasmuch as depending on the texture of product it could peel off the skin even with precaution.
By the way, could you tell the name of the product including Desonide you're using? Desonide is just the active principle.
About the penetration of the product, in general steroids are prescribed with nourishing ointment to assimilate them better.