I was just looking at my Grain farms, and I noticed something odd... at least at first, all of them had the little "workers live in:" note, and I connected them to the appropriate city. Now, for the older factories, the little note has disappeared, and the factories stopped demanding or supplying passengers, and my trains just sit there on platforms costing maintenance. Is this a bug, or a designed feature?
Are you playing Standard Simutrans or Simutrans Experimental? At least in Standard I have never seen this happen before, so I would say it sounds like a bug.
Simutrans Standard- 112.3, R6520. Pak 128.
I've seen factories without workers, but I don't remember seeing factories that used to have workers suddenly no longer having any.
If another factory with higher factory level started nearby, it may drain some workers off.