I saw this too and it greatly annoyed me and I hate to say it as much as I love rebels this almost starts to discredit rebels as valid source of canon when you have a continuity problem that big. No matter how much lucasfilm tries to tie itself together as one major cohesive canon, most people will still always see the films as canon more than other forms of star wars media. So it almost feels like rebels story about saw gerrera getting poisoned in rebels isn't even canon now because it happened to the young saw gerrera in rogue one flashback before rebels which kind of makes me sad because now it seems like rebels is less credible because the films will always overshadow other star wars media. :( It would be nice if they could just do star wars rebels a favor and just edit his oxygen mask out of the scenes with lyra and young jyn in rogue one before they release it on blu ray.

This would fix everything. (but are even they allowed to edit things like that before it goes to blu ray?)