Yeah, there is definitely something odd going on here (as if you needed me to tell you 😉)
I’m guessing that since you are Mac-heads, running Mamp, that you are probably pretty tech-savvy. Could I ask you to try a clean install of 1.6 on your setups and see what you get?
Well, I’m not that tech-savvy but I can let you know that 1.6 does install fine on my MAMP install. 😊