I believe there is a fix - or in fact a procedure to solve this.
Firstly (and this is from the Garmin website):
A new FTP estimate will only be generated outside of a guided FTP test if the following conditions are met:
FTP Auto Calculation is enabled (refer to your owner's manual)
Activity must include heart rate data from either a built-in optical heart rate sensor or from a chest strap
95% of the average power for a 20-minute interval within the ride must be greater than the current FTP setting
https://support.garmin.com/en-AU/?faq=i96M1HykRy4LgvA6bORZC9
It is important that the last two bullet points are satisfied before the device will update with a new and improved FTP. I tried this today and it works.
Secondly, you must connect the Garmin device to your PC with the USB cable, then:
- Go to Garmin Connect in the web browser, go to the top right icon 'Devices & Sync Status' (I'm using an Edge 520), select your device, click on 'Device Settings', then click 'User Settings' and scroll down to Power Zones at the bottom.
- If your ride met the conditions listed above, your FTP will have updated automatically to the figure you achieved in your test. If you want to override this with a different number, you can also do this. Then click 'Save'.
- A message then comes up to sync with your device for the setting to come into effect
- Sync again from software on the PC
- Your next ride/activity should have the correct FTP setting.
This seemed to work for me.