Have you tried with frens.utxo.one? That's a similarly configured, dedicated DM relay. That, or inbox.nostr.wine if you are a subscriber? Testing on non-auth relays isn't really comparing apples to apples.
Discussion
Tried on wss://inbox.azzamo.net and it worked first try
I'm not familiar with that relay? Is it a locked-down, dedicated DM Inbox relay?
On closer inspection, it's not actually doing auth 🤦♂️
Frens fails in the same way as auth. Sounds like nostr:npub1drvpzev3syqt0kjrls50050uzf25gehpz9vgdw08hvex7e0vgfeq0eseet's workaround is what I need to do.
I'm glad you've got a breadcrumb to follow. I hope it yields results for you!