"Dictionary argument "options" without required members must be marked optional" misfires

[domenic]

Minimal repro:

<pre class="metadata">
Title: Writing Assistance APIs
Shortname: writing-assistance
Level: None
Status: CG-DRAFT
Group: webml
Repository: webmachinelearning/writing-assistance-apis
URL: https://webmachinelearning.github.io/writing-assistance-apis
Editor: Domenic Denicola, Google https://www.google.com/, d@domenic.me, https://domenic.me/
Abstract: A
</pre>

<xmp class="idl">
partial interface AI {
  readonly attribute AITranslatorFactory translator;
};

[Exposed=(Window,Worker), SecureContext]
interface AITranslatorFactory {
  Promise<undefined> create(AITranslatorCreateOptions options);
  Promise<undefined> availability(AITranslatorCreateCoreOptions options);
};

dictionary AITranslatorCreateCoreOptions {
  required DOMString sourceLanguage;
  required DOMString targetLanguage;
};

dictionary AITranslatorCreateOptions : AITranslatorCreateCoreOptions {
  AbortSignal signal;
  AICreateMonitorCallback monitor;
};
</xmp>

Error result:

FATAL ERROR: IDL ERROR LINE: 7 - Dictionary argument "options" without required members must be marked optional

(Note the unhelpful line number.)

Very strangely: removing the partial interface makes this go away.