There is no bug, but I think a performance bottleneck. I followed the code, and it simply looks like DelayedJobScheduler isn't able to keep up with the rate of job creations, so the jobs aren't transitioning from Scheduled => Enqueued state very fast.
Using Hangfire 1.6.12 + SqlServer Storage (on Azure P11 SQL Server), with Hangfire Pro 2.0.1.
select count(*) from hangfire.[set] where [key] = 'schedule' and score < 1501247561
Shows me around ~100K however after I killed some of my produces, its enqueuing these scheduled jobs really fast.
I've added ~200K jobs in the last 12 hours.
The strange thing here is DelayedJobScheduler looks very simple -- just an sp_applock + fetch the item with the lowest score from the scheduled set, so my guess is with continuous production of jobs, there's is some other hotspot that is slowing this down (potentially due to transactions?)
I did notice your comment in the other thread about TransactionScope using Serializable transactions by default. Maybe this is the reason.