I went through open tickets in Jira, and at first glance it looks like it can be related to these two issues (although this doesn’t explain wrong last import timestamp):
Is there a plan to prioritize fixing them? If we want ListenBrainz to become more popular, having a working Last.fm importer is a must.
This has now been reported by other people as well: Importing from Last.fm doesn’t import all scrobbles