Breno is right on this. This will not have the desired effect.
You might consider breaking this up into two or three steps:
campaign 1: Lead Source Notes Email
Smart List = Group 1 people who meet whatever it is + Lead Source Notes CONTAINS ZZZ
Send Email X
Campaign 2: Smart List = if Group 1 who Was Not Sent Email X, then go to
Flow where Send Email Random Sample 50, Default = Y which is the 50/50 split method.
Make sure that count in Campaign 2 sounds right. Pretty sure you want Was Not, but check the count and leads who get returned.