Putting aside the API formerly used is anotated beta, the idea we have no explicit control over the values we send is disturbing, so +1.

Did you managed to find a reasoning why the original values is rejected by OpenStack (bug, documentation, etc.)?

The words of the current Openstack Train:

Response format¶
The Networking API v2.0 always responds with the JSON data serialization format. The Accept header is ignored.

But:

curl -v -s -X GET -H "X-Auth-Token: "$OS_TOKEN -H "Accept: application/json; charset=utf-8" -H "Content-Type: application/json" http://xXX.XXX.XXX.XXX:9696/v2.0/ports\?limit\=500 | jq .
 {
   "NeutronError": {
     "message": "The server could not comply with the request since it is either malformed or otherwise incorrect.",
     "type": "HTTPNotAcceptable",
     "detail": ""
   }
 }

I see, https://docs.openstack.org/api-ref/network/v2/#request-format. Thanks!