Shared library is not showing any photos on owner's icloud.com

As you mentioned - The shared library shows up properly in both owner's and other user's iPhone.

There are two owners , you have not created Apple ID account , the second person has created it and shared the library and its photos , between synced devices .

However , your device is synced ( signed in with same Apple ID and password on the same network ) , the actual owner has not given permissions to view photos so you can't access them from www.icloud.com for the third person he can access the photos as permissions are granted .

You might have blocked actual owner who created shared album and the mail address on your Mac Block or unblock senders in Mail on Mac - Apple Support 

As to share albums from your Mac to other person , the invitation has to be subscribed ( Use iCloud Shared Albums with iPhone - Apple Support )

• Turn notifications off: Tap the Notifications button. Tap again to turn notifications on.

See the article If Shared Albums aren't working - Apple Support

See the section - Invites and subscribers

Second scenario .

See the article Use iCloud Shared Albums with iPhone - Apple Support

Manage subscribers and notifications

Select the shared album, tap , then do any of the following:

• Add subscribers: Tap Invite People, then enter the names of the subscribers you want to add.
• Subscribers can add photos and videos to the album. Turn off the Subscribers Can Post button so only you can add photos and videos.
• Remove subscribers: Tap the name of the subscriber, then tap Remove Subscriber.
• Turn notifications off: Tap the Notifications button. Tap again to turn notifications on.

The owner has removed you from the list .

An additional article How to use iCloud Shared Photo Library - Apple Support

Get help with Shared Library

• If you use a device with an earlier version of iOS, iPadOS, macOS, tvOS, or iCloud for Windows, you can't see content moved to the Shared Library. You can see your Shared Library on iCloud.com, or see it on a device that meets the minimum system requirements for Shared Libr
• When a participant deletes a photo or video from the Shared Library, it moves to Recently Deleted. Only the contributor of that photo or video can delete it from Recently Deleted. Learn how to delete photos and videos on your iPhone or iPad or on your Mac.