Howdy Abstractman23,
Thanks for participating in the Apple Support Communities.
Let's check to see if this is a shared album, or an iCloud Link with multiple photos included as these would be two different things. A shared album is a created album that is shared with another user, whereas the iCloud link could have sent multiple photos easier than sending each individual photo. For more details on shared albums on your Mac, check out this resource: What are shared albums in Photos on Mac? - Apple Support
With Shared Albums, you can create albums of photos and videos and easily invite others to view them. The people you invite can view your album from any Mac, PC, iPhone, or iPad. If they use iCloud, they can like your photos, add comments—and, if you’ve allowed it, even add their own photos and video clips to the album. (If your invitees don’t use iCloud, they can still see your shared photos and videos if you set up a public website.)
On your iPhone, open the Photos app, then tap Albums at the bottom. From here, locate the Shared Album section to see which albums are shown and compare with your Mac. For more information with shared albums on your iPhone, review this resource: How to share albums in Photos on your iPhone, iPad, and Mac - Apple Support
Let us know if you find the album on your iPhone in the Shared Album section. If so, compare with your Mac and let us know if other albums are missing or just this one.
Have a great day!