Most countries tax worldwide income of their _residents_ (not citizens). So in your scenario, you'd end up paying tax on the foreign investments if you profit from it when you resume being a US resident. Unless you don't declare the foreign assets, which would be illegal (gotta file an FBAR each year).
There's no loophole here, a large majority of countries work this way, and OECD countries certainly do (although some have different treatments for short term residents). Usually the logic is: if you don't pay taxes in country A, you'll pay in country B anyways. And you'll pay an exit tax when you go from country A to B in many cases, so country A gets their cut.
There's no loophole here, a large majority of countries work this way, and OECD countries certainly do (although some have different treatments for short term residents). Usually the logic is: if you don't pay taxes in country A, you'll pay in country B anyways. And you'll pay an exit tax when you go from country A to B in many cases, so country A gets their cut.