The only team I have opening with spin bowlers, is the BigAnt team. I think it's down to the teams you've downloaded and their balance/stats...
100% not the issue. Not even a little bit, seriously.
If it was due to balancing or stats, it would be across all innings. This is a simple thing - if it's the first batting team's first innings it will not open with spinners, every other innings of the match (even the second batting team's first innings, even if it's first day of the match) a spinner will be one of the two opening bowlers.
It's a bug, pure and simple. When it was introduced I am not sure as i only played as an opener on PC. It's also seems to be a career mode issue - haven't noticed it in other modes, but i'll look out for it - so what the Big Ant team do is irrelevant.
Note, when i say first innings, i am talking about the first innings of the match in terms of the first of four in FC, and first of 2 in list A. So in FC if your side bats first, your first innings they won't open with a spinner but in the second innings they will. If you bat second, both of your innings they will open with a spinner. In List A, if youre batting first, they won't open with a spinner. If you're chasing, the AI will open with a spinner.
Also note, by open i mean one of the first two overs if the innings, not just the first.