So I have the 6S+ as well, and this is what i do before each flight and haven't had any image transmission errors since doing these steps:
It may not work for everyone, but as soon as the transmission error came up I did a little research and compiled these steps to correct my problem. Hope this helps.
-Take case off IPhone (I find it sits in the Mavic controller better)
-Turn on Mavic Controller
-Turn on DJI App
-Turn on Mavic (let it initialize IMU, then calibrate compass)
Then.....
Go into settings on IPhone:
-Turn Airplane Mode on
-Turn Low Power Mode off
-Make sure there aren't any background apps running before returning to DJI App.
-Take off and enjoy