This doesn't solve this problem it is just a workaround I know, but I'll say it anyway just in case someone doesn't know.

I run sorties solo all the time now, you never get the same reward twice in a row. I don't know this for an ABSOLUTE fact but unless I'm absurdly lucky I have never had the same reward twice in a row for like 1-2 months now and I do the sortie every day.

Sometimes they are challenging yes, a few days ago the final mission was defence on the Kuva Fortress, that one was a bitch. I personally enjoy that aspect though but even if you necessarily don't I still think its worth doing alone if you are able.

I'm pretty sure how the reward system works is it won't give the same reward to you twice in a row. However if you are in a group it either picks the host or maybe even picks someone at random in the group and decides what reward to give them, regardless of what everyone else received the day before, then everyone gets that. Which explains why Ayatans are so common.

One quick tip is get Ivara, she is great for soloing loads of different mission types and of course with the Prowl mod she makes spy missions laughable.