.
Has this ever even happened? Why would a traditional bishop use the new rite of holy orders?
To answer your question, no, anyone ordained using the new rite is doubtful, regardless of who performed the new rite.
I think what this means is a bishop consecrated in the Traditional Rite who then later ordained the priest in the New Rite (.e.g. old bishops consecrated before V2 but then later using the New Rite to ordain priests).
I guess there are some who consider the New Rite of Episcopal Consecration doubtful but the New Rite of Ordination valid.
I consider both doubtful and would not go to Confession to a priest either ordained in the New Rite or ordained by a bishop who was consecrated in the New Rite ... except in danger of death with no other option. That last scenario is where doubtful vs. certainly invalid plays in. If you're certain it's invalid, you can't go even in danger of death (it would be meaningless).
I think there are these permutations. I will use Traditional and New as short-hand for Consecrated / Ordained in the Traditional Rite and Consecrated / Ordained in the New Rite.
Traditional Bishop + Traditional Priest = Valid
Traditional Bishop + New Priest = Doubtful
New Bishop + Traditional Priest = Doubtful
New Bishop + New Priest = Doubtful
That's how it shakes out for me.