I believe this is just because the `selectbackground` is only shown when the widget has focus. Try, for example, only selecting part of the text and then tabbing off. Therefore, I think this is not a bug. The only thing which makes me slightly doubt myself is that this behaviour is platform-specific, and not reproducible on Linux, but this could just be down to Windows design guidelines.

IMO, this issue should be closed as "not a bug". If you think this is an issue that should be taken further, please raise it with the Tk team: https://core.tcl-lang.org/tk/reportlist