Do some searching to find many similar issues. A comment recommended DC Rainmaker's blog but be sure to check the comments there on Fitbit GPS issues.
I have the same problem and here's some information that might help.
Even GPS equipped Fitbits seem to use the stride length and step count to calculate distance. People, including Fitbit support, will tell you that only happens when you lose GPS signal but that's not true. As you saw in your case the map can accurately show where you went and the distance can be way off. 10% off is common for me, I'm not talking about it showing me on the wrong side of the street or recording 5.07km instead of 5.09km.
On your dashboard on Fitbit's web page after you select an activity you can export the GPS track using the 'Export as TCX' button near the top right of the screen. Once you have the TCX file you can look at it in a web site (https://www.sunearthtools.com/tools/gps-view.php is one I found via search that seems ok). Alternatively, and probably easier, you can link your Fitbit account to Strava so that your activities automatically upload there. On Strava when you look at the activity you can click on the distance and select 'correct'. This will ignore the distance tags put in the track by Fitbit and recalculation the distance based on the location tags.
When someone challenged the accuracy of the Charge 4 on DC Rainmaker's blog he suggested the user create a test by combining running and cycling to see if the step count really was a factor. I did this and generated a GPS track that recorded no distance traveled during the portion of my 'run' where I cycled, because I wasn't generating any steps, even though the location tags showed I had moved a significant amount (100s of metres).
As far as I'm concerned I have objective proof that Fitbit uses steps instead of GPS to determine distance. This issue was raised a long time ago on their support page and there was no sign that they acknowledge it's a problem or plan to fix it.
