OP was probably not a very technical user, else he/she would've understood that 1. They should've deleted _all_ relevant cookies (in the browser, as well as in the browsers cookie database) 2. There are many 3rd party companies that sell data packs that derive residential IPs from VPN IPs (we use some at work). A trusted/good VPN is a must 3. They probably came via the same User Agent (didn't mention changing browser…
Are you implying that ProtonVPN is not trusted/good? I'm seriously interested in what you know about this particular VPN provider as this is the one OP mentions he's was using.