Notifications by a muted user are still delivered when they edit a subpage of your user talk page (e.g. if they mention you, or comment in a topic you have subscribed to using DiscussionTools). They should only be delivered regardless of mutes when the user edits your user talk page itself (not subpages).
We've noticed this while testing the topic subscriptions feature (T285997).
The code looks like this, with a helpful comment too: (unchanged since it was added in 2017 for T150419: Allow users to restrict who can send them notifications)
// Still notify for posts anywhere in // user's talk space $title->getRootText() === $user->getName() && $title->getNamespace() === NS_USER_TALK
But all of the documentation I could find states that muting only has no effect on your own user talk page:
As I understand, the purpose of the muting exception is to prevent users from muting everyone else and pretending that they don't see messages (the same reason why the "Talk page message" notifications can't be disabled). If that's the case, there's no reason to ignore muting on the subpages.