I have two reads on this.
First: Only Moon Sentences on the sheets are actually in play, in that way. A GM cannot say 'oh, you can't put that sentence down, I have a secret one that actually contradicts it.' If you want to establish something important and big about a moon, consider putting it in play on the sheet RIGHT AWAY.
Second: You can put (seemingly) contradicting sentences on the moon sheet! The two sentences you gave do not actually contradict, in the way that the Wordwrights see the world at least. A Moon can have two sentences that come into conflict. In fact, if a player was to Act by playing their token and saying "The moons are always visible" you could *counter* that sentence with "The view of the heavens always submits to the will of the Rabbit Lord." And even if they did that to Counter one of your own Sentences, you could then reveal your Rabbit Lord sentence to actually SHOW it happen.
You know, if you were writing a Moon sheet for our own Earth, it might have both "The days are always bright' and "An eclipse always blots out the sun." Having both of those on the sheet doesn't mean the sheet is broken. It means that you have put some really fun pieces into play!