Update November 12th, 2020: The issue described below has officially been resolved in iOS 14.2 and it is no longer necessary to completely erase and restore your iPhone in order to repair a corrupted Health database (Apple has removed the support note "HT211865" with instructions on how to do that). It is unclear if this will also bring back the data lost after installing iOS 14.0.
Update November 2nd, 2020: It has been a while since users have last reported this issue. So, even though it is not mentioned in the release notes, it appears to be safe to update to iOS 14.1 now.
Please note that if your Health data was corrupted during the update to iOS 14.0 when it was first released, your data will continue to be corrupted until you manually repair it using the methods described by Apple here. Unfortunately there is no way for 3rd party apps like RunGap to clean up the mess.
Update October 3rd: This issue was NOT fixed in iOS 14.0.1 and seems to affect about one in ten users. Apple has finally acknowledged the issue and and posted a note on "What to do to prevent future data loss". While this may not bring your data back, at least your Health database may be returned to a consistent state and work normally for future workouts.
Apparently there is a really nasty bug in iOS 14.0 released a few days ago and especially Apple Watch users and users relying on Apple "Health" should read this carefully.
The bug does not seem to affect all users - but those who are will no longer ne able to read and write GPS route maps from/to Health. Personally I had several thousand route maps disappear instantly after updating from iOS 13 to iOS 14 and no app is now able to write new route maps to Health. The Health database appears to now have become corrupted and there is no known way to repair it without resetting the iPhone and starting from scratch :(
The bug is definitely in iOS 14 as it affects all apps including Apple's "Health" and "Fitness" apps. So, it has nothing to do with RunGap as such.
When RunGap is unable to read the route map for a recently completed outdoor workout recorded on an Apple Watch, it may "quarantine" the workout for up to 20 minutes while giving the route map extra time to transfer from the Apple Watch to the iPhone. Unfortunately this is necessary in order to work around another old iOS bug causing incomplete workouts (with missing maps) to be made available to 3rd party apps while the map is still transferring (I reported this issue to Apple ages ago).
So, when importing new workouts from Health you may experience that workouts get quarantined and eventually get imported with the route maps missing.
Similarly RunGap will not be able to write workouts to Health including a route map. It may take a little while before the attempt to write the route map is aborted by iOS with an error and then RunGap will clean up the incomplete workout by deleting it and all its data from Health. In that period you may see the workout pop up in Health - but it will be deleted again as soon as the Health database fails to write the complete workout.
In my opinion this is a pretty bad and embarrassing bug and I assume that Apple is busy fixing it and preparing an iOS 14.x update already.
If you experience the issues mentioned above there is still the slight chance that you have not been bitten by this particular bug - but "only" by another less harmfull bug that has been around in iOS for quite some time (also reported to and ignored by Apple for a long time). That bug "only" causes some or all of RunGap's permissions to be reset and can be fixed by following these steps:
1) Check that all permissions for RunGap are enabled in Health as described here: https://rungap.zendesk.com/hc/en-us/articles/222528287-Using-Health-with-RunGap
2) Delete the workouts with missing maps (swipe left in the Activities list).
3) Tap "Resynchronize Activities" for "Health" under "Accounts & Settings" to fetch fresh copies for the previously deleted workouts.
If this still doesn't fix it and you have installed iOS 14, I am afraid that you have been bitten by the nasty sort and will have to wait for Apple (or someone else) to come up with a solution. I highly recommend that you contact Apple support and report the issue in order to give this some priority.
To learn more, try searching the internet for "iOS 14 missing workout maps" to see what others say about this. Unfortunately none of the "fixes" that I found worked for me. I will wait some days before resetting my iPhone in case Apple comes up with a fix. What a bummer!