What I can do help debug this? The digest emails are showing topics from the muted categories for some users.
One thing I noticed, one of these users hasn’t logged into the site for a very long time (say 9 months), the topics send to users email was created in between the time the user last logged into the site and the time the topic was originally created.
User last logged in → 9 months ago
Topic created → 6 months ago
Category muted → 4 months ago