Quick update for everyone … It’s looking like this is caused by bad GPS data.
@templeria has a good example of one problematic activity here - Claire Avery's activity on August 29, 2021 - CityStrides - particularly around miles 4-7.
If you search around the forum for “too much data”, you’ll find many other instances of the maps failing in some way or another. I’ve always figured it was the device running out of memory because too much data was being displayed … I’m thinking now that it’s actually this issue & it’s more common with people who have lots of activities because that makes it more likely for there to be bad GPS.
I’m pretty sure that one of the reasons that the Advanced LifeMap (prior to this ongoing issue) fixed this is that the GPS data is simplified a little bit during the process.
I am currently working on restoring the previous setup.