Hi @Anonymous,
Could you please share a bit more about your PowerApps app's configuration?
Thanks for your feedback, I have made a test and the issue is confirmed on my side. I added a Person or Group type column which filters by a specific SP group in my SP list. Even though, there are some members within this SP group, we could not search them within my PowerApps app via typing a known name/email.
In addition, I could not search people at all within my SharePoint designer when the Person or Group column filters by a specific group. You could take a try to search people within your SharePoint designer, then check if the issue is also existed on your side. If yes, I afraid that this issue may be related to SharePoint itself rather than PowerApps.
I would post this issue to my product team, if this issue is solved, I would reply here.
Best regards,
Kris