Toggles can either be true or false regardless of what is shown in the TrueText and FalseText properties of the control. You can set the default as true or false. If the default property is empty it is interpreted by PowerApps as false. FYI, it is the same for the checkbox control. To make them work in a form you need to do a simple formula
To make them work in a form you need to do a simple formula depending on whether you are saving true/false in your datasource or "Open"/"Resolved".
If(Parent.Default, "Open","Resolved") resolves to if the Parent.Default is true, "Open","Resolved" if the datasource value is (true/false).
Alternatively, if you are saving "Open" and "Resolved" in your datasource, then it would be If(Parent.Default="Open",true,false)
As Drrickryp said, you can only set true and falue values on a toggle.
You just need to modify your default code to this
If(IsBlank(Parent.Default) or Parent.Default = "Open", false, true)
Then set the FalseText to "Open" and the TrueText to "Resolved"