I am having the same issue. It appears that the offending Firmware for the dual is V01.01.0520. This firmware upgrade (if you call it that) did essentially killed the MSX if you are using an IOS device. You can adjust the MSX fusion using an Android device, and I have had mixed results with it holding during cycling the drone off and on.
I did get a response from DJI Mindy on the DJI Forum giving me a username and password to use when I log into DJI Assistant 2 for Mavic. She indicated that V01.01.0521 would be available and would fix the issue. However it was not listed as an available firmware version. The only version available was V01.01.0520 which essentially bricks the MSX functionality.
I have responded on the forum, in PM to DJI Mindy, and sent a support email to DJI, a couple of days ago, and no response at all.