Some builds that are currently running don't appear in the dashboard's "Current Activity" tab

I'm seeing a strange problem on BEAC where some of my builds that are currently running do not appear in the "Current Activity" on the dashboard for some reason.

One detail that may be related or may be a red herring: this plan was set to allow 10 concurrent builds. I reset that back to 1 while there are 2 currently in progress.