No bug here. This is how SwiftButton is implemented. You can access its internal label and modify it as needed: SwiftButton.xLBL. And you can change it with the designer. It will also work.
@Theera
Here is how to do in the way @Erel proposed (taking what you was doing in another your work)
B4X:
Dim myFont As B4XFont
myFont = xui.CreateFont(Typeface.LoadFromAssets("JSGiat-Bold.otf"),62)
SwiftButton1.xLBL.Font = myFont
SwiftButton1.xLBL.TextSize = 30
This is how to change Font and TextSize from code to the original SwiftButton.
I understand that it's not intuitive, but if @Erel does not want to update the SwiftButton this is the way to go.