Hi, replying to my own thread here in case anyone reads it...
I found a solution on another thread: https://community.fitbit.com/t5/Charge-5/Charge-5-has-stopped-tracking-my-distance/m-p/5632653 (thank you to MarreFitbit who provided the answer). The solution is partial - I've improved it, below!
MarreFitbit's solution is to toggle off the automatic stride-length feature. FitBit is set by default to automatically calculate your stride length, based on data compiled when you track runs or walks (distance tracked on GPS divided by number of steps recorded by the pedometer/accelerometer) but apparently, something got broken (I say this because it used to work properly/perfectly). Now , if your app is toggled to automatic*, it is possible, like in my case, that the distance on your tracker will only show GPS-tracked distance - not your total distance covered based on number of steps and your average stride length.
If you turn the automatic setting off, you'll need to manually set your stride length for it to track your distance at all, but, at least for me, this did mean that my on-wrist-tracker started counting miles as well as steps again (even when it's not tracking a specific manually-activated workout).
The solution is not perfect, because it means that you have to use a personally-defined average step-length (one for walking, one for running) but it's better than not tracking distance at all.
NB, my app had a non-zero value for the running stride but showed no data for the walking stride, so that got me wondering if that could be related to why it wasn't tracking when I wasn't actively 'working out'...
An improved solution is to toggle stride-length to manual, input manual average figures to BOTH walking and running stride-length fields, then toggle back to automatic. THIS WORKS and makes the best use of the available data based on current app functionality. I don't yet have enough real-world data to know if the app is updating my stride-lengths automatically yet but will hopefully come back in the future to confirm. It's my conclusion that the mileage-counter was broken because the walking stride-length was set to zero, which must have happened when the app was updated from FitBit to Google. Time will tell if the automatic updating of walking/running stride-length works, once there's a base-value in there to update...
To access stride-length settings and fix the issue, click on your profile pic at the top right of the app > go to FitBit Settings > scroll down and choose "Exercise" > tap on "Stride Length" at the top > toggle "Set automatically" to 'off' > make sure there's a sensible value* in the field for walking stride length and running stride length > toggle "Set automatically" back to 'on' and exit back to the home-screen.
*In case you don't know how and you want to work out your stride length, you need to either use the values from a recent workout to calculate it (this is a good way to do it because you'll typically be using a big sample for your average) or walk/run a specific number of steps (e.g. 10) and then measure the distance you covered. To calculate stride length, divide the total distance covered by the number of steps.
FitBit asked for my stride length in inches so don't forget to convert your distance to inches (or whatever unit FitBit is asking for) before you divide by your number of steps or you'll get a really weird answer that will throw your distance out massively.
I hope that helps someone else out there!
