Hi @Gti182 , Please add below code on Items property of manager combo box:
Office365Users.SearchUser({searchTerm:Self.SearchText})

Add below code on the Manager_DataCard1 data card update property:
{
Claims: "i:0#.f|membership|" & DataCardValue3.Selected.Mail,
Email: DataCardValue3.Selected.Mail,
DisplayName: DataCardValue3.Selected.DisplayName,
Department: "",
JobTitle: "",
Picture: ""
}

EDITED:
And on the OnSuccess, add Refresh(SPLISTNAME)

Then Save and publish the form. Refresh the SharePoint list and try again. It worked for me.
-----------------------------------------------------------------------------------------------------------------------------
I hope this helps.
Please click Accept as solution ✅ if my post helped you solve your issue. This will help others find it more readily. It also closes the item. If the content was useful in other ways, please consider giving it Thumbs up.👍
Thanks,
ANB