I’ve created a custom game that allows only D.VA and turns off her ult, primary fire, and defense matrix. There is a setting to force D.VA to respawn in mech which works after you’ve been killed, but if you fall off the map inside your mech, you respawn as baby D.VA instead. Since the ult is disabled, she’s unable to recall her mech and must jump off (or get killed) again to respawn with mech.
This seems to happen regardless of map and at the very least on both deathmatch and team deathmatch modes (maybe others).