So it could be the case that one Backgrounder process is enough to meet the needs for now and one is down. Or it could be the case that both Backgrounder processes are processing but the status is not showing correctly.
If you capture a set of logs, each Tableau Server process will create its own log file. If one Backgrounder process is down, I would think it's log file would be much smaller than the other. It might show some type of error/exception near the end of the log file and that's when logging stopped.
Another way you might be able to tell is by looking at the "Background Tasks for Extracts" Admin view. On this view there's a "Backgrounder" drop down and it should list out both options if they've both been processing extracts. The "Background Tasks for Non Extracts" Admin view has the same Backgrounder drop down so might be able to tell if the one process had been working and suddenly stopped.
As mentioned earlier, it could be the case that status is not being reported correctly and this can occur for example if the Tableau Server is running on a machine with dual NICs. This Help article goes into more detail in how to address that:
But if none of that helps I would recommend to open a support case.