Could be the reason, though I remember multiple strong forms after week 1. But the form drops for the several players who didn't play full 48 minutes all dropped 1 or 2 levels.
The training issue bothers me more, since the situation looks way more blatant to me. I have a 19 year-old 6'9 player with atrocious JS, respectable range, average driving and handling. He had 4 trainings of IS from last season, which definitely trains JS as well. I don't see why he wouldn't pop from 1 training. I don't think his DMI changed, either. All this happens with a level 9 trainer.
Now I am not sure this isn't much ado about nothing, but because of the previous issues with JS for Forwards, I thought I'd post it here just in case.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."