I want to know why I get an event ID from captureMessage (and send feedback with captureFeedback / legacy captureUserFeedback), but the feedback does not always appear in Sentry.

If you have an event ID, you can use Explore and search by the following:

Make sure you use the full 32-character ID. If nothing shows up, the event was dropped - for example, due to sampling, beforeSend, inbound filters, quota, network issues, or something similar. See Why is my event missing in Sentry even though I have an event ID?.

Also check User Feedback in the product UI to see whether the feedback itself is there. Avoid firing the same captureMessage text repeatedly without a page reload. The default Dedupe integration can drop duplicate events, which means later feedback may have nothing valid to attach to.

Prefer Sentry.captureFeedback({ message, associatedEventId }) (or your platform’s equivalent). Feedback can also be sent without an associated event if you use the widget/API that way - please see User Feedback.