multiple percent signs in single prerequisite - revisited

[Peter Apian-Bennewitz]

Dear folks,

after 2 decades or so of happy gmake usage, I've hit a wall, apparantly:

A rule like

     %.mp4: %/%.mov
or
    %.mp4: foo_%_%.bar

substitutes only the first % while looking for matching prerequisites. - why ?

The same question remained unanswered in 2004: 
https://lists.gnu.org/archive/html/help-make/2004-06/msg00105.html
Same topic on stackoverflow&Co doesn't shed much light on this.
Checked with latest freshly compiled make-4.2.93 .

Is this RTFM or function-as-designed, and if so, why ?

TIA1e6
Peter