I can't talk about all strains but most of the auto-strains aren't stable yet. (especially nirvana seeds had this problem, you can read it all over forums)I had wierd results outside last year with the Snowryder feminized variety from Sensi, it's nor a dwarf nor a "full" regular plant, somewhere in the middle, anyway, it's classified as an autoflowering strain. I planted them last year outside in mid May, they got way bigger than they were supposed to and showed no signs of autoflowering, I ended up chopping them, I did have one that matured in late September and the bud was great but I guess they got accustomed to a regulare photoperiod and didn't autoflower, whatever.
Maybe your seed batch was unstable.. (last years?)