Link to the play https://xfl.myfootballnow.com/watch/1613#279263

Looks to me based on the play by play and video that the defensive player fumbled the ball into and out of the end zone, so shouldn't I get the ball back at my 20?

The game must think it's a safety because the play began with you having possession of the ball, and if you fumble out of the back of your endzone then it would be a safety. However, if the recovering defensive player fumbles into and out of your endzone, I believe it's still their ball at the spot of the fumble. I don't believe your team can regain possession without one of your players physically recovering the 2nd fumble.