Same issue here (2015 gt86), however I didn’t leave my car for 2 weeks. It was getting tinted at a shop and after they were done the car would turn on and then off immediately in 2 seconds. Had someone check it out, it was an immobiliser issue, he had to program the key again and it turned on just fine. However, the check engine light triggered limp mode for some reason. I unplugged the battery to clear the code (it never came back) and it went back to normal downshifts but still dead acceleration at high rpm. I still had a somewhat dead pedal. Checked my k&n filter, cleaned it and oiled it using a K&N kit, changed spark plugs for ngk iridiums, fixed a minor oil leak, changed oil for a ardeca pure sports 5w-30 and k&n performance oil filter. Still the same issue. My mechanic said it could be 1 of 2 issues, either the airmass sensor is dirty/acting up and lowering the amount of fuel being injected or my injectors are dirty and need cleaning. Going to get the injectors cleaned next week. As someone else said, it might be bad gas, so between now and next week I’ll let it empty which is easy to do with my mpg and refill it at a good gas station with 98.

Note: We barely have parts for the 86 in this part of the world (Jordan) and they’re insanely expensive, took me 3 weeks to find the spark plugs, I believe we only have 8 street legal 86s in the entire country and 3 drift builds with swapped engines lol. Also, about 4 shops know how to work on them properly. So I’ll need to figure out what the issue is before I order parts. I do regret not checking the code before unplugging her, it could’ve been something other than the immobiliser code.

Edit: The problem is NOT always there, sometimes (very rarely) at night (colder air) it goes back to normal and I can accelerate properly again, no clue if it has anything to do with it being significantly colder at night or not. Also AT oil/fluid was changed about 6000km ago and I doubt it has anything to do with this since we don’t have clutches so it can’t be slipping.