Can’t invite people to my existing iCloud Shared Album on my iPhone

I just noticed something interesting: on my iPad Pro (12.9-inch) (2nd generation) which is still running iOS 17 (ver 17.7.8), I can add participants to this album. There’s a ‘Invite people’ button below the list of participants, so I can add people on that device.. And I can also add people through the Photos app on my iMac.

But on my iPad 9 running iPadOS 18.5 and my iPhone running iOS 18.5 that button does not appear. Something happened in the update to ver. 18 on the mobile devices to disable that function under certain conditions. I just can’t figure out what those conditions are.

Yes, I had a conversation with Apple Support about it, and we set up a remote control session and they saw the problem but couldn't figure out what's going on either. I have many other shared albums, and with all the others I do see an 'Add Participant' button. I have around 800 photos in this shared album, and about 70 participants, but that shouldn't be a problem. So I don't know. It was working fine until recently and I'm not sure what happened to cause the problem. And very odd that I am able to add participants to that album on my Mac but not on the iPhone or iPad.