The signature is format_float($float, $decimalpoints=1, $localized=true, $stripzeros=false)

If you call format_float(100.0, 0, true, true)

then you get 1, becuase the zeros get stripped form 100. Oops!

Since MDL-73954, this can cause problems in the quiz. See https://moodle.org/mod/forum/discuss.php?d=432864.