welp https://f-droid.org/en/docs/FAQ_-_App_Developers/#will-my-app-be-built-from-source

I’ve published a new release. Why is it not in the repository? When we detect a new release, it may take a few days to make it into the repository as the build process has several steps and runs only once a day. Before the build has completed, the monitor page for your app will list it in F-Droid Monitor - Need updating. As long as the text under Versions stating “The current (recommended) version is xxx (version code yyy)” shows the version numbers corresponding to your latest release, we detected it and the APK should be available soon. Just give it some time.

Another reason could be that the app failed to build. You can watch the build process on F-Droid Monitor - Running and the previous cycle in Build.

Also https://gitlab.com/fdroid/wiki/-/wikis/FAQ#how-long-does-it-take-for-my-app-to-show-up-on-website-and-client

How long does it take for my app to show up on website and client? Your MR has just been merged, but the APK did not show up immediately? That’s normal: the APK must first be built on the build server (happens automatically when the next build cycle starts), then signed (manual step), then a new index must be created and deployed.

a build cycle currently can take up to 72h (hard limit)

apps are manually signed after that (if not reproducible), and then uploaded next build cycle starts after signing is completed

So if you’re lucky, it takes 3 days (new build cycle just started minutes after the merge). If you’re not-that-lucky (merge happened a minute after a build cycle started), it will take 6 days. If you’re very unlucky, Murphy visits in between with some problems… So don’t panic before 7 days have passed, please. 😉 Please also note that the website is updated asynchronously – so while the index might already have reached your local client, website might still be a bit behind.