This is clearly a bug in macOS Photos app.
For me (macOS 14.5), MacBook Photos wouldn't even show that there is a Shared Library (Settings / Shared Library would show "Enable" button even though the Shared Library is in use). Out of desperation I rebooted the machine, and now Photos at least shows that Shared Library is enabled in Settings, but the Shared Library appeared empty and no syncing appeared to be ongoing.
Out of desperation I went to settings, iCloud and enabled "Download Originals to this Mac". The "Downloading x Originals" text appeared, but after a while photos started to pop up appearing in my Shared Library as well! The process is still ongoing, but it looks promising now.