I am seeing the same issue, and testers are reporting that they are not able to download the latest build in TestFlight. i wrote to Apple support and got this response: "The dash symbol under the testers status references the tester was successfully added to the testing group, but no builds are currently available. When a build becomes available, the tester will receive an invitation." I tested this out: I created a new External Testers group, added many of the users that have the "dash" status, then added the latest active build. After refreshing the page several times, the users are still in the dash status. We need a solution for this.

This appears to have cleared up - I am no longer seeing the Status of "-", and have stopped getting reports of users not seeing updates.

This has returned - I have a distribution group of 115 testers that has a valid, active build in the Testing status. Over half of the users in this group are now showing a status of "-" and I am getting reports that some cannot download the latest version (the TestFlight app is requesting a code). Removing then re-adding the users does no good - they come back into the list as "-".