coromonadalix, it's not an issue of worrying. it's an actual phenomenon that happens on my desk right now, with a cheap USB tester. The moment I attach another grounded cable or instrument to the PCB I'm working on (in addition to the USB that's powering it via the tester), the current reading on the tester drops by anywhere from 20% to 50%. that makes sense too, because the return current now has two paths, some of it goes via the tester and gets measured, and some via the other ground path and isn't. that's why high-side sensing is important.
As for a homebrew solution, I can definitely design one, I design such circuits all the time in my professional work. I just want something off-the-shelf that works correctly out of the box in this use-case, without setting something custom up.