The whole ''beginners should all start with seat harnesses'' isn't the best advice anymore. As long as you can find the correct size and correct fit harness there are no reasons even younger and lighter riders couldn't start on a waist harness.
The main 3 reasons waist harnesses rise up and become annoying/uncomfortable are:
1-Harness is the wrong size/fit or of too poor design/quality/tool old
2-Kite is being kept at 12 the majority of time
3-Kite is TOO BIG, i.e. learner is overpowered
The last 2 points are a very common sight with the majority of learners, especially lighter ones, even when taking lessons...
Christian