Why can't Microsoft validate my publisher domain?

When trying to validate our publisher domain I get an error message complaining about an JSON schema mismatch.

Verification of publisher domain failed. The JSON file found at https://www.awaro.com/.well-known/microsoft-identity-association did not match the expected JSON schema. [qbw9PqEbu5ho8MwTcgYL8t]

In a browser I can download the JSON file without any problems. The Content-Type header is set correctly. These are the headers I get:

HTTP/2 200 server: nginx date: Fri, 20 Sep 2024 10:41:37 GMT content-type: application/json last-modified: Wed, 18 Sep 2024 15:00:17 GMT etag: W/"66-62266106c3f7e" age: 4304 strict-transport-security: max-age=63072000; includeSubDomains; preload x-varnish-cache: HIT x-cacheable: YES x-content-type-options: nosniff x-xss-protection: 1; mode=block content-encoding: br X-Firefox-Spdy: h2

Is this a problem with the file extension? We are hosting the file under https://www.awaro.com/.well-known/microsoft-identity-association.json as required by the instructions but the error message does not show the .json extension. Our website provider does not allow files without extension to be hosted, so I cannot change this.