No it's not normal.
It is a self switching relay that monitors the main battery voltage and only allows charging of the leisure battery when the main battery is fully charged. To me it sounds like there is a fluctuation of the main battery voltage around the point that the relay is supposed to switch. I am not sure, but you may find an adjustment inside to set the threshold level which could stop this. Failing this, you could just bin it and use an ordinary heavy duty relay taking a trigger from the alternator so it will only ever switch when the engine is running. I had a similar smartcom relay once and I found it didn't isolate the batteries for whatever reason and as the leisure battery went down, it dragged the main battery with it! I binned it and went with the tried and trusted simple option. No problems since!
