Hi Teh,
Below is my thought and you could try to explain it to your customer:
Firstly the Advanced Find editor is built with legacy web client UI, while marketing application is totally built with UCI.
We can still browse marketing entities in legacy web client via "Power Platform Admin Center > Settings > Product > Behavior" by disabling "Use Unified Interface only" option,
however, it wouldn't make sense;
for example, if I open a marketing email record, it will tell me that I need to switch to marketing application to edit this type of record, and designer is in "error loading control" status.
Then when I open event registration lists, I found that the "Check in registrations" button doesn't appear.
So the reason could be: "Check in Registration" function might not be implemented in legacy web client.
To be more specific, we could take Lead entity as example:
If we want to add a lead into a marketing list, pop up windows are different in Advanced Find and UCI:
In UCI, a confirmation dialog will be opened if we check in a registration record:
However, due to "Check in Registration" button is not available when we select an event registration record in legacy web client,
so it could be said that there is no corresponding dialog or window for event check-in confirmation in old UI.
In Oct of this year, legacy web client will be deprecated, and "Advanced Find" will be also expected to get aujustment.
Regards,
Clofly
