I've had the same experience. After two on-site interviews, an assessment, and a series of tests. I got offered 2k/month for a full-time job (despite my current job making about 25% more at the time). I declined on the spot, then got a mail with the same offer (including the standard "we are confident we're giving you a good offer". I mailed them that I declined, got no reply, and got called the next day (during work) if I could please respond to the mail.

Weirdest interview experience ever.