Why are there entries in JobQueue table that have no corresponding Job table entry?

I am using Hangfire SQLServer storage. I am seeing many entries in the JobQueue table that have no corresponding entry in the Job table. On the dashboard Jobs page these are being displayed as Job expired and in the Job detail page as Background job ‘113998’ has expired or could not be found on the server.

Is this expected? These entries are not being removed by the ExpirationManager.

The database has a mixture of 1.5.8 and 1.6.0 clients and servers.

I noticed that I did not have a server running that processed these particular queues. Now that I have run a server, these entries are being removed (dequeued). However this is happening very slowly (maybe 1 per 30 seconds) even though my QueuePollInterval is 10 seconds. I have enabled TRACE level logging, but cannot see any entries in the log related to the dequeuing of these jobs.