I mean it is kind of a theme in Final Fantasy games that the ‘gods’ (either real or claimed) are flawed, manipulative liars. So it wouldn’t be out of tone for them.
And I mean Luna is in a position that would make her pretty easy to groom by a god if that was their inclination. She becomes fairly isolated after her mother dies with the Empire keeping her captive, Ravus throwing himself into his revenge-plan, and her mother not there to guide her. The youngest Oracle in generations is both an impressive feat but also a dangerous position because she doesn’t have the experience that comes with age to not just trust what she is being told blindly.
And the thing is no-one, not even Noctis, has enough information that they would be able to see what is actually happening. If Noctis was actually able to spend time with Luna while she was alive in Altissia he (or Ignis) might have been able to clue in that the gods might not be trusted. But, of course, Luna dies before that has a chance to happen doesn’t she? Once she’s dead it’s not like she can really go back and change things then - her part in it at least is set in stone now.
And I mean Episode Ignis proves that, much like most other Final Fantasy games, if you actually challenge the gods/destiny there is another way. You just need to fight harder for it.
Do I actually think the gods in this game are lying and being harsh to Noctis + Luna? No, I just think they see it as the easiest way to achieve the necessary ends and are worried if they offer the alternative the heroes will take it and fail. However I will say there is enough evidence to read the situation as being this way as well I just don’t think it was in the contemplation of the creators (who knows though, Episode Luna might change my mind on that point).