Inconsistent response from Spring Security HTTP Basic between GET and POST

Why do I get a 401 from the POST but always get what I expect from the GET?

###
GET http://localhost:8080/user/3
Authorization: Basic username password

<> 2021-07-21T205654.200.json
<> 2021-07-21T205029.200.json

###
POST http://localhost:8080/user
Authorization: Basic username password
Content-Type: application/json

{}