Well, I redid the test by first creating a pool with Standard_D8s_v5 and then trying to edit it adding the flex node Standard_D8as_v5, which is a real necessity for us and is a viable option according to documentation. However, got the same result. Response 200 OK, but the pool doesn't have the flex node configured.

I also tried creating a pool from scratch using the REST API and passing an instance not compatible. It created the pool and the flex node configured, with said instance. After trying to create the pool again passing the flex node "foo", I got a INVALID_PARAMETER_VALUE and a message that the valid value is actually a white list of any instance. It doesn't check if the flex node is compatible to the instance on the pool. So we're back at the start, if anyone has any other suggestion