Sorry error in description should be:

{"host":{"id":53,"errors":{"ip":["Does not match selected Subnet"]},"full_messages":["IP address Does not match selected Subnet"]}}

when only updating/changing subnet. (host already had an ip adress in the old subnet)

and

{"host":{"id":53,"errors":{"ip":["is invalid"]},"full_messages":["IP address is invalid"]}}

when trying to also update ip to an empty string to see if it would trigger the auto-ip.

If I update subnet to the same subnet as it already is then the call with not give an error, but thats because nothing changes.

Can you try setting the IP to a null value at the same time, rather than an empty string? "ip": null in JSON terms.

Get the same errors as before :(

curl -s -H 'Accept:application/json' -H 'Content-Type: application/json' -k -u admin:changeme -X PUT -d '{"host":{"subnet_id":7,"ip":null}}' https://foreman/api/v2/hosts/53 {"error": {"id":53,"errors":{"ip":["is invalid"]},"full_messages":["IP address is invalid"]}

curl -s -H 'Accept:application/json' -H 'Content-Type: application/json' -k -u admin:changeme -X PUT -d '{"host":{"subnet_id":7}}' https://foreman/api/v2/hosts/53 {"error": {"id":53,"errors":{"ip":["Does not match selected Subnet"]},"full_messages":["IP address Does not match selected Subnet"]}