Hi @Anonymous,
I think the issue here should be related with Record mis-match.
SharePoint lookup fields (Like Choice, Lookup, and Person or group type) is considered as record type under PowerApps, you should be able to find the proper record elements by checking the lookup field advanced options:
It should be the following:
{'@odata.type':"#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser",
Claims:"i:0#.f|membership|michael@wiXXXX.onmicrosoft.com",
Department:"",
DisplayName:"Michael Shao",
Email:"Michael@wiXXXX.onmicrosoft.com",
JobTitle:"",
Picture:""
}
The Office365user.MyProfie() would give the following record elements:
Office365Users.MyProfile().Department
Office365Users.MyProfile().DisplayName
Office365Users.MyProfile().GivenName
Office365Users.MyProfile().Id
Office365Users.MyProfile().JobTitle
Office365Users.MyProfile().Mail
Office365Users.MyProfile().MailNickname
Office365Users.MyProfile().Surname
Office365Users.MyProfile().TelephoneNumber
Office365Users.MyProfile().UserPrincipalName
Office365Users.MyProfile().AccountEnabled
We could change the Default property with the corresponding record (People field), then replace the data with the following:
{'@odata.type':"#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser",
Claims:Concatenate("i:0#.f|membership|",Office365Users.MyProfile().Mail),
Department:Office365Users.MyProfile().Department,
DisplayName:Office365Users.MyProfile().DisplayName,
Email:Office365Users.MyProfile().Mail,
JobTitle:Office365Users.MyProfile().JobTitle,
Picture:""
}

Tested from my side, the corresponding user would be added without selection.
Let me know if this works in your side.
Regards,
Michael