For an example of this see info.magnolia.security.app.dialog.field.SystemLanguagesFieldDefinition. It has a method getOptions() which uses MgnlContext.

When I configure this field in the JCR I never pass through SystemLanguagesFieldDefinition#getOptions(). So that is why we don't see this MgnlContext issue when configuring the dialog in JCR.

On the other hand, when configuring in yaml, when I save the dialog I pass through SystemLanguagesFieldDefinition#getOptions() for whatever reason.

The dialog is still usable but you do receive a warning when the dialog saves. A possible workaround would be to move the loading of options to the factory class or just ignore the warning.