I would take it as a signal that you got along well with your old employer even after you parted ways the first time. That's a good thing.
If you had quit or had been fired from your current job, I would be afraid the same might happen with my company and you will be back with the old company as your fallback at the smallest problem, but a contract running out is a good reason to change employers and I cannot find anything bad about that.
I would not consider it a red mark. But hiring people is not exactly hard science, other people might think different.