I have a Microsoft Store app stuck in 'publishing' for well over 12 hours.

For MSIX and add-on submissions, the documented publishing times are:

• MSIX apps: publishing typically completes within a few minutes, and customers generally see the app’s listing within about 15 minutes after publishing starts.
• Add-ons and other app submissions: publishing is expected to complete within 24 hours for new apps and updates that include package changes, and less than one hour for updates that only change listing details.

Once a submission has entered the Publishing phase, it cannot be cancelled and the release date cannot be changed. The only available actions are those exposed in Partner Center (for example, waiting for completion, or later changing availability after it is in the Store).

If a submission has been stuck in Publishing for well over 24 hours, this is outside the normal documented window. In that case, the next step is to contact Microsoft support via Partner Center so the submission and account can be checked, since there is no self-service “nudge” or cancel option once publishing has started.

References:

• The app certification process for add-on
• The app certification process for MSIX app