Maybe the whole question is a mistake. Isn’t it possible to say that when the Gemara says that Hashem decides the Zivhugim for every fetus at conception, it is referring to ALL babies – including non-Jews? The gemara does not specify frum Jews or even Jews at all.
If so, since HKB”H knew that the Ger would be Jewish by the time he would be ready to get married, Hashem picked out a nice Jewish girl for his Neshama.
Just proposing a possible answer. I don’t know what the Gedolim have written about it. I have no problem being shown to be wrong. I have been right a few times in my life.