As an oldskool Mavic Mini owner with no FN button, this is obviously not a solution BUT there is a (very clunky) workaround.
I'm now on 1.4.8 (in-app update prompt) and running on an Android 10 device. If you go into photo mode, you can lock the exposure by the tap and hold on the yellow circle, right? So, what you do is tap to bring up the yellow circle but DON'T hold to pick the exposure. Now quickly go to video mode, and the yellow circle will briefly turn into a yellow rectangle. NOW you can hold that and it will lock the exposure.
Obviously this is clunky as, but it does work. Also, to unlock the exposure you should be able to simply tap anywhere on the screen or, if this doesn't work, switch back to photo mode and then back to video mode again and it'll clear the exposure lock as it always has done.
Let me know if this works for you guys and gals as well.
Regards,
Ad