I have a similar warning from Site Health, and evidence seems to back it up:

Your author URL https://roosenmaallen.com/author/admin/ does not return valid JSON for application/activity+json. Please check if your hosting supports alternate Accept headers.

erose@lorge:~$ curl -i https://roosenmaallen.com/author/admin/ -H "Accept: application/activity+json" --http1.1
HTTP/1.1 200 OK
Content-Type: application/activity+json
Transfer-Encoding: chunked
Connection: keep-alive
Keep-Alive: timeout=15
Date: Sat, 28 May 2022 17:32:59 GMT
Server: Apache
Link: ; rel="https://api.w.org/", ; rel="alternate"; type="application/json"

erose@lorge:~$ curl -i https://roosenmaallen.com/author/admin/ -H "Accept: application/activity+json"
HTTP/2 200
content-type: application/activity+json
date: Sat, 28 May 2022 17:33:09 GMT
server: Apache
link: ; rel="https://api.w.org/", ; rel="alternate"; type="application/json"

erose@lorge:~$