Maybe close the velcro behind the seatpost and run a ziptie through it to secure it to the seatpost.
Some friends have also had problems with the clamp on quick-release beam racks tearing up shorts.
I tore up a lot of bike shorts after getting a bike fit done / moving the saddle BACK about 1cm. So this is not OP's problem but still relevant to the thread subject.
I was destroying a pair of $100 bike shorts in less than 40 miles. Spent over $1,000 dollars in 6 months trying all different brands and models of shorts. The cheap shorts were more durable than the high-performance wicking shorts.
Gore makes a style of bike shorts that has a more durable fabric on the inner legs and sitbone area. But it's a compromise, the durable fabric doesn't seem to breathe or wick away sweat as well as other brands.
I finally bought a new seatpost that had a smoother clamping mechanism AND was a setback post. It moved the clamp back just enough to avoid contact with the upper inner thigh. The clamp had a tiny amount of black lint accummulated from the barely-noticeable fabric contact while pedaling.
I also found that my bike saddle undercarriage had broken, leaving a tiny rough crack on the outer edge when I was sitting on the saddle. I replaced the saddle too.
Problem solved.