I’m foolish to procrastinate, must have been tired, this is easy to show :

a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c

So between each a and b, and b and c, there is an unbreakable space, so there should be breaks only between c and a. `fill-paragraph' honours that.

When looking the html version, which, modulo some extra time, majordomo accepts to let pass, you will see linebreaks after a « a » or a « b ».