Hey Carl,
The issue that was shown here is obviously a crimping fault. Normally when using the correct crimping pliers, wire should not slip out of the contact when stretched. Of course there are limits of what a crimped joint can withstand.
That is why where a cable is a moving cable, there always should be a strain relief for the entire cable. The contacts inside plug each have a little strain relief of their own for each wire (the crimping pliers, if chosen correctly, takes this into account), but there should always be a strain relief for the entire cable at both ends, if it is a moving cable.
This is also recommended inside drag chains at both ends, where strain relief is often omitted in a DIY environment.