We've implemented a hotfix for this issue as of 6pm PST on 9/26. We're keeping our eyes on it, and we're hopeful this will solve the problem.
We've identified a server related memory issue that occurred this weekend and are working on a patch to get it resolved as soon as possible. When it occurred, it would have appeared to riders that they were riding alone for a bit. Some may have also had some interruptions to their automatic Strava uploads as well. This was different from other occurrences of "disappearing riders", as the usual cause is related to internet connection issues on the user's end. This one was on us.
We're now aware of the issue and should have a fix very soon. We apologize for any inconvenience that you may have experienced.