[Gameplay] Unable to queue up non-full premade team that effectively guarantees primaries.

This past weekend, I was playing League of Legends with three others. I really wanted to play mid (I've been going ADC for what feels like forever out of necessity). I noticed we had the following setup:

Player A: Fill Player B: Jungle/Top Player C: Top/Mid Me: Mid/Secondary

I went ahead and originally threw Bot as my secondary, but hoped to get Mid. We got into champion select (and subsequently had dodgers) multiple times. Each time it assigned me "Bot." Finally, when we had one of our own failed to accept the match (he didn't notice someone failed to lock-in and it threw us back in matchmaking), I took the opportunity to change my secondary to Top.

Now, my personal priority (usually) is: Mid > Top > ADC = Jungle = Support, with a preference not to Support since I tend to do better than a lot of my friends at carrying.

As soon as I did this, I heard the party leader begin saying: "What the... I can't queue!" He originally remade the group and assumed it was because the "Fill" guy only had one role selected, but as he was explaining this, I switched back to "Bot" as my secondary. Boom, fixed. He instantly queued us, and again I was assigned ADC.

Amusingly, the random, ended up taking Jungle. This chained the group until making me Bot lane, thus three of us got our secondary roles.

So, I thought about it, and I was thinking that I regretted switching, but it wasn't allowing him to queue us up when I had Top as my secondary. Is this intended? Effectively, our queue would've required the random to fill either Bot or Support, with our fourth taking the other. In order to give us each one of our two selected roles, it would've had to give Player B Jungle, and C and I would've gotten Top/Mid (swapping, if for some reason it gave us our secondaries rather than primaries despite being in a premade). I can't easily test my hypothesis right now, but I suspect it wouldn't have allowed him to queue if my secondary had been Jungle either, since it would have the same result.

What's the deal? Shouldn't we just get a longer queue time to find someone who wants Bot/Support/Fill?