These pictures show the propane tank and the restraint in its cabinet. The second picture shows a closeup view of the restraint in question.
This is used on many RVs when there is a single bottle to be supported in a cabinet. The other bottle is on the other side of the rig and is strapped securely using the same hardware.
To loosen, one grabs the left side of the silver clasp and flips it outward to the right - one might keep their hand over the clasp to prevent a "violent" movement as it snaps loose. The loop on the other side is supposed to keep everything together as it snaps loose. The silver loop on the other side of the clasp is removed from the black metal hook on the strap and opened to remove the propane tank.
I was thinking that the strap was coming loose at the clasp. This is usually caused by it being too loose.
If the strap is coming loose at the wall, then that's a different problem. I have not taken a close look at that, but I expect that the hole might be worn too big for the fastener and new larger diameter hardware needs to be used, or use something completely different like the loop and hook strap someone mentioned prior. The strap still has to be fastened to the side wall somehow.