In my mind the "sel" tag is special, but it nevertheless complies with the tag priority rules identically as any other tag.

For me the current behaviour is OK.

Regarding the man page, in my understanding section "THE SELECTION" of the [text] man page does not really say anything about the priority of the "sel" tag.

I don't clearly see where "THE SELECTION" seems to imply that the "sel" tag could always have the highest priority, except perhaps when it says that "the -inactiveselectbackground option for the text widget is used instead of -selectbackground when the text widget does not have the focus"? OK, here we could maybe append "(depending on the priority of the sel tag compared to the priority of other tags)" or something alike better phrased.

As a final note, "THE SELECTION" could say a word about -inactiveselectforeground since it already explains -inactiveselectbackground.