I tried to resist commenting, but sometimes I can't help myself
I saw that thread, but saw no evidence for a HAZ related issue. Possible mechanisms for a HAZ related failure could be "cold cracking" where the HAZ gets heat treated by the welding process and forms very hard martensite. This can lead to cracking, generally quite quickly after welding, so i'd suspect a failure soon after it was made. Only happens with certain types of alloyed and carbon steels, not the steel i'd expect it was made from.
Alternatively, the tube could be heavily cold drawn (cold worked), and the welding process softens it around the weld. If the stresses were high enough in normal use, it could then fail by fatigue, but I feel it unlikely as the actual strength loss by annealing in the HAZ would be small in a mild steel.
There has obviously not been any testing to prove this, and I don't suspect many people would let me chop up their wishbones!
The wishbone has a huge stress concentration factor at the weld, due to the shape of it. The failure will almost always occur at the weld, but that does not mean the weld is at fault per se.
I'd go along with fatigue failure due to excessive stiffness / misalignment of the bush / crush tube too short etc, unless there's any evidence to show otherwise.
Andy