KroFau timeline
1. They were on talking terms as late as the 24th December 2023
https://youtu.be/NhSeOqnVkoQ?t=31652. The earliest I can find of them not being on speaking terms is this tweet from March 23rd where she blatantly ignores Fauna despite replying to basically everyone else even those who replied after Fauna (she liked Shioris despite not replying to it)
https://x.com/ourokronii/status/17703490165278641644. On Feb 3rd In Kronii's first outer wilds stream, Fauna typed "Outer Wilds yay!! I hope you enjoy it Kronii!!" Kronii "missed" this message, whether intentionally or accidentally is unclear, she did "wait a second" right before Fauna sent the message and was fixing something so its definitely possible this was unintentional
https://youtu.be/JqDlkndKJjA?t=138 (her message appears onscreen under the yellow superchat)
This gives us an easy timeframe, the "incident" occurred after the 25th of December 2023 but before March 23rd 2024
Events Happening between these dates
>Kronii gets some form of surgery (Early Jan)>Kronii plays Outer Wilds (Early Feb)>EN goes to Japan, Fauna and Shiori seemingly didn't go as their signatures were printed out and glued on (Early Feb)>Fes (16th March)I can't think of any other events although you may be able to
The obvious conclusion (in my opinion atleast) is that Kronii and Fauna had a small argument about Fauna not going to Japan which got a little too heated and led to Fauna saying something she later regretted.
To me Kronii always seemed like a 19th century romantic wearing the skin of a stoic jester in order to cope with the harshness of the world and Fauna always gave me the impression that she had an extremely sharp tongue that she very rarely used so its believable to me that Kronii could choose to act like someone doesnt exist for an entire year because of an out of place sentence and its believable to me that Fauna could find that specific sentence.
Combining this with a high stress situation for the both of them, Kronii's surgery, FES, Kronii going to Japan, Fauna either getting hounded for not going to Japan (may have planted the seeds for graduation) or maybe she had a legitimate reason for not going to Japan which would also be stressful.
These two personalities in a high stress situation was always a powder keg just waiting for a spark, perhaps Fauna asked some questions about Kronii having played outer wilds before (Kronii was pretending to not have played it outer wilds before despite obviously playing it) which Kronii may have been sensitive to, Kronii may have been seriously asking Fauna about her Japan absence or joking around with her about not going to Japan (a topic which Fauna is notoriously sensitive about), the issue is that this discussion has to end in such a way that Kronii deems it appropriate to spend an entire year acting like Fauna doesn't even exist while Fauna has to walk away not even knowing they arent on speaking terms before clumsily wishing her well on a sick tweet only to get straight up ignored.
I dont personally believe we will ever know for sure and with Fauna's departure I don't see any situation in which KroFau rises from the ashes.
TLDR: KroFau incident happened between Dec 26th 2023 and March 23rd 2024, This was during a period where all ENs other than Shiori and Fauna were in Japan