Loss of an erection in such situations is mostly due to the pure distraction element of the tickling.
You mention that this happens to your converted partners. Odds are these partners are drawing on the submissive aspects of the tickle play for the jolt that gets them off, over the tickling in and of itself. As a result the tickling is a significant distracting stimulation for them, that is not directly associated with arousal in their sexualities. It's mot unlike how one might react to a spanking if pain play was not in your make up.
The solution to this is rather simple. You have two hands. One to tickle with, the other to provide hand job-sort of stimulation with at the same time. Direct cock attention will keep them hard pretty much through any tickling (Thank you male arias design) and has the bonus of providing a reinforcing connection between the tickling and the hand job which over time might lead to some conditioned arousal from tickling alone.
Myriads