Given that it makes little sense for Somnus not to have faced nasty consequences if he really had blatantly rejected the will of the Astrals, there's actually a much more coherent option as to what Aera actually heard.
Imagine this: Aera goes to the Astrals to ask which of the brothers should be king. Instead of actually being given a name, she is told that the identity of the king must be determined by the brothers themselves. Somnus tells Aera that he would be happy for Ardyn to take the throne, and sends his soldiers out to tell Ardyn he has been chosen. Then, when Ardyn gets there, Somnus says that he was actually the chosen one and provokes him into a fight to prove Ardyn's corruption and ensure that no one can reasonably think that Ardyn would have been a better choice.
Somnus wouldn't technically be disobeying the will of the Astrals, so he wouldn't incur their wrath, but Aera would still have every reason to feel betrayed by Somnus' actions and interfere with his plan. It'd also explain why she didn't seem to be happy about the Astrals' choice -- she would have known that not being able to give a name would complicate things.
My personal theory is that neither of the brothers were actually chosen and instead Aera was told that the Caelum line would have to wait until the real chosen one came about. Both of the brothers were only ever going to be guards to the crystal until the true chosen one came along. Basically Noctis was the only one the crystal ever chose otherwise either Somnus or Ardyn would've been given providence right there on the spot and been allowed to absorb the crystal's light to destroy the Starscourge 2000 years ago but that didn't happen. Instead Ardyn was flat out rejected and Somnus was never given that ultimate power. The line was basically tasked with waiting until the actual guy meant for all that power showed up.
Part of why I think this is because all of the Lucii are given titles at some point which they are remembered by. These titles often pertain to their most defining trait. Somnus was "The Mystic" due to his magical prowess on the battlefield, Ardyn is "The Accursed" due to his cursed existence, Regis was "The Father" due to being Noct's father, and lastly Noctis was the one and only person to formally receive the title of "The Chosen." No other king but Noctis has ever been known by this title despite there being a lot of talk about Somnus or Ardyn being chosen by the Crystal. I strongly believe that this is because they never were chosen at all, and merely tasked with waiting and protecting the crystal.
Makes sense too if you think about it. Noct's the only one of the 114 kings with no actual blood on his hands as far as we know. He's the only one who, genuinely, has never been a piece of shit or put others at risk to further his own agenda. Even Regis who was a fairly benevolent king eventually had to get his hands dirty and risk a lot of lives for his son. Not Noct though, Noct may have been spoiled but all the death around him was always a direct result of someone else being bad. None of the tragedy around him ever permanently broke him either. He was someone both Somnus and Ardyn could never hope to be.
Ardyn eventually broke under the weight of tragedy and Somnus' hands had way too much blood on them. Not to mention I don't think either of them would be so ready to lay down their lives like Noctis at the end of the day, especially at the ripe old age of 30. Ardyn ultimately wouldn't be able to let go of Aera imo considering she was the only source of love in his life and Somnus would've probably been too attached to the world to go given how much he covets the throne. Basically the Astrals needed someone who could never be corrupted and someone who would never turn away from his destiny. The 3 other Kings that we know of in FFXV came up short in one aspect or another at the end of the day and who knows about the other 100 kings and queens which is why Noctis was the only choice ever possible.
That's just what I think though and I could be entirely wrong but it would be an interesting spin on all this.