I've reproduced a case that matches this in dark mode. The default button has white text and a translucent white background, so both blend into a white window. I've logged the bug here.
Could you confirm whether Windows is using dark mode, which Studio version you're running, and whether this happens in the designer, the running application, or both? That will help me confirm whether you're seeing the same issue.
For now, giving the button an opaque BackgroundColour should keep it visible. Unless you've explicitly set its TextColour, Objo will automatically choose black or white text for contrast.