Apple Mail Blocked Senders email lands in Inbox NOT Trash

Refer to Block senders in Mail on Mac - Apple Support.

With blocked mail you have two choices: mark as blocked but leave it in your Inbox, or move it to the Trash. It would appear you have chosen the first one.

I understand, if the spammer uses a variety of "from" names adding each one to Contacts isn't a viable solution.

If Mail isn't doing what it's supposed to do then it's a bug. There is a reporting procedure which may only be available to developers, and if I can duplicate what you're experiencing I'll file one on your behalf.

Other than that you can use Feedback: Feedback - Mail - Apple. It has a bug reporting option also. If you describe what's going on in sufficient detail it will at least get their attention. If Apple can duplicate it, it becomes likely they will address it in a future macOS update or upgrade.

Unless you or I are overlooking something obvious I think all you would get from a Genius Bar is a shrug.

Back to your question, since Mail's built-in features don't appear to be working. Try writing a Mail Rule that explicitly sends those messages to Junk: Use rules to manage emails you receive in Mail on Mac - Apple Support. If you want to get really creative you can augment those Rules with AppleScripts.

Of course it would be best if "blocked senders" worked as it should.

It's not supposed to do that. One potential workaround that I have heard of is to add that sender to Contacts, and block it. See if that works.