I wouldn't be surprised that the first crimp is faulty due to the indentations being too near the centre. Part of the width of the counductor crimp zone is wasted on the endstop as you say, the remainder is on the very end of the conductors where the strands are likely to splay and escape, or even be forcibly ejected as the tube becomes funnel-shaped at the edge of the crimp zone. I would liken it to the very end of a conductor pinched halfway under a choc block screw.
I do also share concerns about the mechcanical durability of yellow crimps and the current rating of a completed joint relative to the size of conductor. I would only use products from a proper manufacturer with published performance and dimensional specifications and approvals. For critical applications as per Darkwood and Davesparks I prefer uninsulated crimps with a hex die - mechanically stronger, more consistent and reliably gas-tight crimp pattern and easier to inspect for successful installation.