Having two places where a joint can slip is generally considered less secure than one. Theoretically with the toe arm design with a locking feature on each end you only really need one to be secure to prevent the rod from turning on the threads so in OPs case both ends failed to secure the arms adjustment.

The toe arm has two clamps as well... if you compare it to the LCA, the black barrel of the toe arm is the same part as the threaded rod with the nut on it between the arm and the rod end. Instead of clamping into the arm, it clamps to the other rod end. Same effect though, and like someone else mentioned, for the toe to change both clamps have to be loose enough for the barrel to rotate, in theory you only have to stop one end from rotating to stop coarse adjustments from happening. They run two so that the movement of the rod end at the other end can't cause a tiny change in toe.

Only one clamp was loose and the amount the end can turn on its own is more than enough to get some good toe "adjustment".

Update:

Only got one session before something in the drivetrain gave up the ghost! But, the steel bolts held the toe link clamps, yay :)

Also got a msg from SPL saying they'll replace the links with pretested links if the problem re-occurs. Nice.

Next test will be on Z214 slicks (and fixed drive train).

Good news, and bad news :(

What part failed?

Left axle. Not bad for 4+ years of tracking the car hard with an SC. Should I upgrade or stick with stock?

Upgrade. Sticky tires will destroy axles. I've already broken one.