tl;dr – First put the phone in airplane mode. Next, sync your activity to the phone. Lastly, take airplane mode off and sync your activity to Polar Flow.
I ran after flying and hadn’t taken phone off of airplane mode; happy coincidence. Here’s the complete rundown that works for me.
First, put the phone on airplane mode, then open the Polar Flow App. A popup asking for permission to turn Bluetooth on appears; say yes. Next, sync your M400 with your phone as usual.
On the phone, you will be able to tell that your activity has been uploaded (to phone, not to Polar) by looking at the daily activity window. This window will have noted your activity in the doughnut chart. Your activity will NOT show in the training portion of the app. Close the Polar Flow app. This completes step on.
Lastly, just take airplane mode off and open the Polar Flow app again. Now just wait for app to syn to Polar site. After the usual wait, your activity will show up in the training portion of app, success!
Once the activity has been recorded (you will see the circle graph
Polar M400 syncc Fail Android Workaround
Two-step. First sync watch with Android phone in Airplane Mode. Watch will state sync fail… let it be. Run will NOT show up in Flow app in phone. Lastly, turn Airplane mode off and sync app again (so that it syncs with Polar service this time.)
Mario Talavera Writes 
Most amazing. So happy you ran into this before I did 😉
I narrowed it down to NFC. If you disable the NFC radio, the M430 syncs over BTLE, and Smart notifications work as well.
Neat. Such a good watch. Good find!
Mine says unable to do in flight mode?