I have the opposite problem from the one discussed in this thread from 2015

I have a classmate who is registered and paid for an event. Yet there is no icon next to his name in the class list and there is no mention in his profile about his attendance.
It's Classmate Robert Galanti - the event is "Saturday Evening - pay by check"

Here is how to fix it...

Event Planner
Edit
Transactions
Edit the transaction for the Classmate with the problem
Click the pencil icon
Check the “Guest is a Classmate” checkbox
Save the changes.

Thanks Kyle, that worked. I should have tried that before posting but I didn't know that someone could register as a "guest" without being the guest of a classmate. I guess he just mistakenly registered as his own guest, without registering himself as himself.
I have a headache from just thinking about it

He isn’t really a guest, but the system doesn’t try to look up the classmate unless the person claims to be the classmate. I guess that is because sometimes a classmate can purchase tickets for someone else.

The same problem could also happen if you have classmates who married other classmates. The classmate who purchased the tickets might not have said that their spouse was also a classmate.