i think thats normal, all builders will repair stuff they just built.

Actually factories have an option of whether to repair the unit being built before starting the next unit, but since strider hubs are nonstandard I don't know if that option applies to them. Either way it doesn't drop its repair order even if the strider moves out of its build range, which is difficult for an AI to recover from. (at least it requires a bunch of extra crap to track the unit so that unitFinished can be used to signal the next build job)

Also, I think I've seen strider hubs continue repairing the old strider even when there was a visible order for another strider, although that's difficult to confirm.