I have had similar issues, and found that it is sometimes necessary to "scroll down" to find more "towns" than the primary you first discover.
In other words, you may find the town name listed by itself, and right below that (keep scrolling!) you may find it listed again in conjunction with another town. That "in between" unincorporated area sometime shows up that way, and the address on the street becomes valid once you've told the TT that it's not in the city proper.
As an example, I have a friend that lives outside of Louisville, Colorado (mailing address). He's actually just outside of Louisville a bit in unincorporated Boulder county, and his street doesn't even appear in the Louisville listings. However, when I dropped down just a bit (quite by accident) I discovered a listing for Louisville/Lafayette that took in the unincorporated areas near those two towns within Boulder county. His street did show up there.
I've also notice what you describe -- an address doesn't show up for a particular street when it's outside of town. This can also happen if you select the wrong town in a metro area where the towns share streets. The software seems to sometimes understand the boundaries of the cities, and will only take the numbering to the city limits.
Poke around in the town/city list and see if there isn't something that gets his street covered. It may not be where you first suspect. I can understand the TT's brain damage in this regard -- the residents are equally confused! -- since the overlap between postal addresses and incorporated cities and unincorporated rural areas can get very confusing. My old town was Niwot, Colorado, but our mailing address was Longmont, Colorado since they delivered out of the City of Longmont. Niwot isn't even incorporated -- just a name on the map that was originally just a rural railroad stop. There are unincorporated areas of the county where apart from a mailing address, you can't be sure what city they might have for their association.