I'm a little unsure on your exact question, but I can say that in terms of a 'copyright strike' relevant email (usually from a company to your ISP, which then your ISP will notify you).. this is not something you need to worry about using a VPN. Try a site like ipleak.net and open the magnet detection in your torrent client (with VPN on) and check that ipleak shows the IP of your VPN and the VPN DNS server(s). It's a great way to test.

In reference to your ISP contacting you previously, I would think this is in relation to a noticed amount of traffic on your account. In the fine print of many ISPs the 'unlimited' plans are still governed by a fair use policy, where through inspection of your accounts usage they may have noted significant upload and/or download quantities, hence they have contacted you. Did they specifically mention it was from torrents? As far as I know they should not be able to inspect the traffic if it was masked by VPN, they should only see the amount of data on a throughput. My country's ISPs are commonly known to warn customers about excessive usage when on an 'unlimited' 4G mobile network plan. But it does vary from country and ISP.

I hope this helps.