I’ve had this issue too on my wizard in hardcore. I took a pic of the buffs to show… I have the shield pylon effect already and then the enchantress cheat death went off while I still had 53 seconds of pylon effect left.
I play on Xbox HC ladder. This needs to be fixed. 
I know this is a necro, but it’s happened to me 4x this week. Active shield pylon and BAM, proc.
One thing I notice though, is it seems to occur almost immediately after I dash. The same with insta proc’s that occur about once every three rifts where I don’t have a shield pylon on. From what I see, it’s almost always after I dash. And the difficulty also doesn’t seem to matter here. I’ve had it proc on low level as well as high level GRs.
This happened to me twice on the same day. Both times in greater rift with enchantress while under effect of shield pylon. This is on a season 25 demon hunter both times. Imagine the rage if someone lost a hardcore character to this bug coz cheat death was on cooldown or the follower was dead… but then the shield pylon fails! It brings your health down despite being in shield pylon.
One of my two procs during shield pylon was immediately after a vault. The other was just strafing in GoD build. Both of my procs was while fighting Dune Dervish mobs (tall spinning dudes)… in 2 seperate grifts.