Organization default model can fall back to the platform default
A new organization no longer stores a concrete default model. When none is set, the platform default applies, and the Models and Organization settings pages label that state Platform default with the model name.
To go back to the platform default after choosing a model, select the platform default option, or send "default_model_id": null in PATCH /v1/orgs/{org}. Omitting the field leaves the setting unchanged.
See CLI: models, #3114 and #3321.