bug#79497: fmt breaks lines when it does not need to

Hi,

With the attached input file and coreutils-9.8, I see these
results from the 'fmt' command, in the en_US.UTF-8 locale
on a GNU system:

$ fmt -w 12 -g 12 input.txt
Grüß
Gott.
Здравствуйте!
x=(-b±sqrt(b²-4ac))/(2a)
日本語,中文,한글
$ fmt -w 13 -g 13 input.txt
Grüß Gott.
Здравствуйте!
x=(-b±sqrt(b²-4ac))/(2a)
日本語,中文,한글

Why does it take a width of 13, in order to keep the first two words
on a single line? That line has width 10.

I would expect that already 'fmt -w 10 -g 10 input.txt' keeps these
two words together.

Bruno

Grüß Gott. Здравствуйте! x=(-b±sqrt(b²-4ac))/(2a)  日本語,中文,한글