
Yes, but there is probably no way to fix that as it is not possible to manually set the okurigana at present.
I'm adding it to the list
[@moderator] Sentences with the wrong reading for kanji

And it's beteer to be chaged to:
ひと月に一度
or
月に一度

Better as in "This is a way to avoid a wrong okurigana", or as in "This is how a native would write it anyway.?

Both.
We write and say:
ひと月に一度, ひと月に一回, 月に一度, 月に一回, 一ヶ月に一度,
一ヶ月に一回, 毎月一度, 毎月一回
And I think that 一月 is not wrong also, but when we saw 一月, we imagine January first of all.