Same issue here. JJ McCarthy resets after every game or getting upgraded. I might as well sit him and use Darnold at this point.
While annoying it doesn't impact your rookie's development.
Even while their dev trait is hidden they still get XP at the rate of whatever it will be revealed as. For several iterations you can actually tell what people's dev trait will be in practice by comparing it to other people's XP gains because of that. The only impact it has is if someone is a superstar or X-factor rated player they can't assign abilities until that is revealed, but McCarthy is star not SS or above.
It's weird though, I got the glitch, but after the current season next year everyone's traits were revealed right after the draft. Once I was in my 3rd season of franchise it started working correctly.
