That stinks. I did just check the wayback machine and it looks like, as recently as 13 December, the Norwegian webpage indicated all Premium fares had lounge access.
Current page:
https://www.norwegian.com/us/travel-...premium-cabin/
13 Dec page:
https://web.archive.org/web/20181213...premium-cabin/
Sometimes, when they change access rules, they grandfather in passengers who bought tickets before a certain date, etc. However, in this case, it appears to be whatever ticket you have at the time of travel, regardless of the policy at the time you purchased the ticket. You can keep trying, but I don't think there is anything illegal about this (I AM NOT A LAWYER, so take it for what it's worth) as lounge access rules are distinct from the fares terms & conditions and the airlines contract of carriage with the passenger.