I was in creative and I killed the Ender dragon with potion effects enabled as infinite, I went through the portal after killing the dragon and the potion effect was cleared for no reason without milk, without the clear command, etc. I went back and tested it again and had the same results. I tried it with effects applied for a few minutes and it still happened. Unsure if it occurs in survival.

On further testing it does occur in survival. This is a really weird but because of the fact it ONLY happens on the end exit portal. Not the entrance portal, or the End Island portals.