Now I have it in bits I can see how constant use would break the hinge.
The alloy pins on aluminium tube put a strain on the plastic hinge which is trying to lever the two halves of the hinge apart. The welded lugs give way on the left hinge. The right hinge has only one alloy pin to bare on so the plastic has cracked around that pin rather than lever the two halves apart.
The pins hold the protector square with the locating lugs for stowage, otherwise it would be all floppy and awkward to locate.
If, like me, you push the protector down to make sure it's firmly seated, that puts a strain on the hinge through the alloy pins.
So something gives eventually.
Think I will now not use the protector. Silicon the metal plate over Hall sensor and stretch a cargo net at the required level.
And just make sure nothing can move around in the boot.