It may appear that you have a weak skin layer under the glans, which may be the coronal ridge's skin. This skin layer shed quite often, and new layer of skin tends to be young and easily influenced by the pressure. The underskin layers (fascias) harden up over time and prevent it from happening. However...
This may be the trigger. Heat soften the skin layers and easily cause edemas. However, my concern is the time under negative pressure. Are you using the pump straight for 20 minutes between 7 and 8inHg? This may be the trigger. If you're doing it straight, edemas will form. Even with me. Break it down to smaller time period. Pumping for 3 to 5 minutes, perform
SSJ or reverse jelqs, and resume pumping. Straight pumping under pressure that high will cause edemas since the fluid cannot backflow or drain from the penile body along the veins or the lymphnodes.
Cycling the pressure as in dynamic pressures? Yes, that is correct that if the pressure raises too fast and drops too rapidly, it will cause edemas. The discussions have been talked about quite extensively. This is why we indicated to crawl during the raising of the pressure, and takes 10 seconds or more to release to the pressure from high to low. This stabilizes the pressure in the chambers and the fascias. Think of a human body going deep sea diving. Going up too fast from the depths will cause rapid gas expansions, and going down too fast causes rapid gas compression. Divers, or submariners, goes down and up slowly. Same for snorkers that interswitched with quick dives. I took the experiences of snorkling and quick shallow dives to bring to PE, and always chimed in about slow expansion and release during pumping.