Transplants don't cause stress. That's a myth unless you absolutely destroyed the rootball intentionally while up-potting.
This is urban myth that you need to start an autoflower in a small pot.
read good info on cocoforcannabis about up-potting.
I can tell you in 5 years and 400-500 up-pottings, so far, i have yet to see transplant shock. The percent risk is so tiny. This is just one of those things people repeat because it 'sounds' good. Like 48h of darkness increases trichomes -- sounds sciency but absolute bullshit. flushing before harvest even if there is no problem in your rootzone - sounds sciency but has no effect on flower composition, and do they flush plants outside? no, because that's absurd. Defoliation for no reason - not a good idea. this hobby is riddled with faux-science that sounds good but is absoluetly wrong.
So, this "transplant shock" is a myth. What we do is technically "up-potting" which is not stressful for the plant if you don't molest the rootball while doing it. Transplanting plants outside invovled cutting roots and re-planting. this is very stressful, but not the same thing.
go check out cocoforcannabis. dot com. their info is some of the best out there with the least amount of bro-science i've found, so far. the grow guides and articles in dr photon's corner are must-reads.
i'd wager this is due to tempermental autoflower. your fertilizer was too hot or left it in the starter pot too long before up-potting. If the roots "think" they are confined, it'll flower. (think being a strong word, but a feedback loop is involved one way or another.)
people say autoflower are easier, but that's just not true. you should know how to grow a photoperiod seed-to-harvest in a healthy way before you grow autoflowers. Ensuring darkness isn't as hard as most people think. that is the only thing that is easier with an auto because you don't have to worry about it. everything else is more difficult if new to growing pot.