It would be strange because resets are usually things that last a long time, but in this case 9 days have passed by the day counter, is it possible that there was a reset and it was simply forgotten?
It's not so much a reset, as it is amnesia, or being placed on autopilot. Resets generally follow a cycle, return to a starting point, and affect everything within Kumon-mi, with the notable exceptions. This is actually just what happens to Akira a lot, he's put on autopilot while he's elsewhere or unconscious and then when he returns it's [amount of time] later, and he was seemingly acting normal in many cases. Assumedly only select people or the beach trip itself were affected, as well. Another case of someone tampering with people's minds/memories and changing things for whatever reason.
Regarding Himawari, she almost certainly has plans for or to help Ayane. Right now she seems to be fighting for self-preservation and to aid Ayane and Akira, at least. No idea if her motive here was because Ayane would have been reset, or what other danger she was facing.
As an aside, not sure if this was supposed to actually be a full week+ beach trip or not. I haven't looked at the code, but I'd be interested in checking if something unintentionally caused the day count to increment by eight instead of one. I thought it was a weekend trip until I read that, but could have just been wrong. And the interference adding a week would be weird. Oh well.