When we select ride with it should always show the ride with rider on the information panel on the right as you can loose them in large groups - then you can always see where you are in relation to them.
Yes, I could see how that is frustrating. You join to ride with someone and then you loose them in a crowd of riders.
I have a question, is the other rider on Zwift mobile app? You can “friend them” on the app. Then when you login and they are also on it will show your friends at the top already logged in and riding so you can select to “join them.” You will be allowed some assist time to catch up with them and their speed after login.
If you loose them, since you are friends on the app it will display a friend icon next to their name on right hand side of screen during riding. Should be easier to spot them in relation to you.
Hope that helps and thanks for your suggestion.