I think the issue is more with Last.Fm than Mac or Spotify.
Have you disconnected your account in the settings and logged back in? Last has done this before where you just needed to re-enter your password to get it to connect.
I link Last for my iphone and desktop apps and have noticed it doesn't always sync the connection when you start playing - even when your password has been re-entered. I don't know if this is because I am switching platforms and it's confused to the source - but just to be safe I always restart my app on whatever device I want to scrobble from.
😞 That must be really frustrating. You are on the latest update to High Sierra too (10.13)? I have not seen any functionality change with my installs so unfortunately I am not sure if the glitch resides in Mac or elsewhere. Can you scrobble from a mobile device?
Yes, it has done that to me in the Spotify app as well! When I last got disconnected from LAST.FM I just reset my password on their site and then immediately reentered it in Spotify too. Sounds like a weird unnecessary workaround, but it has kept me synced ever since and I have been on all updates of High Sierra since release.