FIU-Arkansas St. ruling

[TriggerMN]

This play happened in Tuesday night's game. I am not a football official anymore so while I know the rules are different for 4th down fumbles, I don't know the specifics well. Anyways...

Team A has the ball 4th and 1 at team B's 31 yard line. A1 completes a pass to A2 who runs to B's 27 yard line, then is hit and fumbles the ball backwards to B's 35 yard line. The ball is picked up by A3 at the 35, and carries it to B's 10 yard line before being tackled. (The ball is dead at the point A3 grabs it, right?)

The ruling on the field gave the ball to team A, 1st and 10. I do not remember if A received the ball on B's 27 or B's 35 though.

I found this ruling curious, as I thought B would get the ball. Team A gets a 1st and 10 because they made the line to gain, even though they fumbled it backwards? Any clarification is greatly appreciated.