Wheel bolts coming loose

I use winter tyres on steel wheels on my 2011 Octavia L+K estate, usually from October to April and the original alloys for the rest of the year. 

The steel wheels  were sold as "use original bolts" and look identical to the steel spare in the boot but are not VAG parts.

In the last two years when I swap back from winters to summers the bolts come loose again after a few hundred miles. I always make sure the bolts are tightened to 120nm using a torque wrench. 

What I think is happening is that although the steel wheels have the radiused mounting face that matches the bolts it does not extend as far as on the alloys so after six months of gunge there is a partly corroded and pitted surface attempting to tighten to the alloys when I swap back. Over time this is getting to be more of a problem.

I wondered if anyone else has had a similar problem?

I'm thinking the only long term solution is a new set of bolts to keep exclusively for the alloys and keep the originals for the steel winters or am I overthinking this?