Since you asked for "any idea", I'll assume you are stumped and offer a very low probability possibility. Fungal nose infections can cause an acidic discharge that discolors the nose in a somewhat similar pattern to that:
Unlikely that this would be the only symptom, and probably wouldn't cause the hair loss out that far, but did look a little reminiscent of the way the light skin extends around the flare of the nose.
Update : We took Ellie to the vet a few days ago. They suspect demodex, but did not find any with a tape pull. They didn't want to scrape the sensitive area.
We are treating for all of the usual causes, Nexguard tablet for demodex, topical antibacterial/antifungal/steroidal cream, and systemic antibiotics (cefpodoxmine) .
Ellie is super healthy so demodex is strange to be a cause, but hey she had pyometra before the age of two so she is already a special cookie.
The hair loss appears to be getting larger but of course we'll have to give it a few weeks to really know if one of the treatments hit the cause. Besides the hair loss and what the vet thinks is some inflammation (darkness on nose to face area) she is very healthy and active not skipping a beat.
I haven't been treating her with any parasitic/insect preventatives as I don't find them necessary in the winter time here. Maybe the demodex mites took advantage of the reprieve and held a little party.