Me too. Already filed 2 bug reports. Thought it might be connected with the transition from water to ground, but it could be level related. Didn’t check it yet, but as it happens when walking from one part of a map to another (touching) part, it could be that it happens when your level is adjusted. I hope they will address this problem quickly, as it is very irritating, although it reminds me of GW1 where the minions had a limited life.
Just tested it and it is definitively scaling related. Summoned a minion when I was level 6 and walked to level 4 area and boom, same happened the otherway around. Submitted a 3rd report informing Anet about that