My patching seldom, if ever failed but I usually try to use tubes of the right size, though partner's deep V rims and broad tyres make getting long-stemmed, broad tubes a challenge sometimes!
As LWaB posted in reply #12 upthread, tubes can fail at the junction of the valve stem reinforcing patch and the normal tube, if overstretched.
I see undersized tubes as a 'get you home' rather than a long-term, solution.
Having said that, if I'd used an undersized tube, I would not change it until I had another visitation or was about to embark on a long tour or Audax.