I have an Omas Paragon 2003 Bleu Venezia.

The piston knob operates normally, in that it turns anticlockwise to lower the piston to expel air, and turns clockwise to retract the piston and draw up ink. With the exception that once the piston has been full retracted (and the piston knob is flush with the trim ring), the piston knob will continue to turn clockwise without any additional resistance so I don't know when I've finished filling the reservoir. I don't mean to say that it spins freely, but if I were distracted, I could conceivable keep turning the knob for an hour and not realise that the reservoir was already full!

Is this a problem that could cause me concern, or just an idiosyncrasy of this pen?

There is no leakage that I can discern, the pen does not drip or burb ink when filled, so I don't think any of the seals have failed.