Android is brokeThe only way to
get a yellow box is to exit to the start
screen, hit the "GO FLY" blue button
- then you see the yellow box (and
lock icon) for a split second; gotta tap
it quick to get it to stay a bit longer.
Then long-press to change between
AE locked/unlocked. Once you do
nothing for a full second or so, the
yellow box goes away; start over to
get it back. Alternatively, you can map
the double-tap of the Fn button to
flip AE lock. Useful, but you get no
indication of whether the lock is on or
off.
icon on the screen at all times, so you
know if you are locked or not.

YEP! What he said..

Hope that will be fixed soon... works in photo mode.

I just tried Photo and yes there is some functionality.  It has to work for video as that's probably 90% of what most users capture with the drone.

Hello there BlairwareVision. Good day and thank you for sharing these information here at DJI Forum. I will forward these information to the designated DJI department for further evaluation. Thank you.

Similar question in Fly forum, same response -

Hope the next update fixes the buggy 1.4.0 update. In the meantime for AE Lock, you can try swiping diagonally like for defining a marker for Spotlight. Once the green box shows, tap on the green x to close it and the yellow brackets will appear briefly. Tap and hold inside the brackets until the AE Lock message appears. To cancel AE Lock, just tap the screen. It's working this way on my Samsung Galaxy A71 with Android 11 (edit: on a MA2). I wished it worked the same as on iOS though.

Pappy3 Posted at 4-20 23:32
Similar question in Fly forum, same response -

Hope the next update fixes the buggy 1.4.0 update. In the meantime for AE Lock, you can try swiping diagonally like for defining a marker for Spotlight. Once the green box shows, tap on the green x to close it and the yellow brackets will appear briefly. Tap and hold inside the brackets until the AE Lock message appears. To cancel AE Lock, just tap the screen. It's working this way on my Samsung Galaxy A71 with Android 11 (edit: on a MA2). I wished it worked the same as on iOS though.

Nice temporary workaround.  Thanks for sharing.