We have an add-in deployed via centralized deployment that is available via OWA but not Outlook desktop. It's been 36 hours since it was deployed.
We are using M365 Apps for Enterprise, Outlook desktop version is 2202, Build 16.0.14931.20764.
I tried signing out and back in to Outlook desktop, several times, no change.
A key difference between Outlook versions 2202 and OWA is support for Requirement Set 1.12. If your add-in manifest requires that, then it will not load on that older version of Office.
Documentation here - https://learn.microsoft.com/en-us/javascript/api/requirement-sets/outlook/outlook-api-requirement-sets?view=excel-js-preview&tabs=xmlmanifest