I updated my Ionic last night to Fitbit OS 4.2 (71.6.19) with no issues.
I used BT and it took a long time, but that was expected.
Today it recorded my walk and no issues with the GPS. It recorded heart rate, steps and distance.
But about one year ago, after the upgrade back then, I had the problem you faced.
My IONIC was brought back from the recycle bin by performing a factory reset.
Try resetting your ionic first - (This is a normal reset, not a factory reset)
Press and hold the left and bottom right buttons together, until the Fitbit icon appears, then release. This will not erase your un-synced data or IONIC settings.
If that does not work try a factory reset. But per several threads, you should contact fitbit for help in advance.
Please do note that a factory reset will erase un-synced data and you will lose all settings that you might have changed.
After factory reset you will need re-set up your Ionic completely.
On Ionic, open the Settings app ( ) > About > Factory Reset."
If you cannot do the factory reset from the Watch, the following are the steps that worked for me:
Remove the Ionic from your account.
Delete the bluetooth pairing.
Press and hold all buttons on the device.
Wait until the Fitbit logo disappears, then release the bottom right button.
Wait until you feel a vibration, then release the other buttons.
You will probably see a red X message, just ignore that and setup the device again.
I follow this from a post from another fellow in this group.
When I paired my IONIC, It had to download the latest firmware again, and I had to set all the parameters I use.
After that, for over a year, all firmware's updates have worked and I have not seen the issues again.
