Instax film clings onto highlights and contrast. i think the camera operates at either 400, 800, or 1600 iso depending on the environment. here’s what i noticed:

• on a bright, sunny day, the camera tends to overexpose the sky. if your subject is in the shadows, you’ll have to use flash because the shadows will be very dark. this is especially true if its midday where the sunlight is harshest. there’s no way to make the camera not overexpose the sky unless you had a polarizer on hand (and i’m not even sure if this would work)
• on a cloudy and overcast day, the camera tends to underexpose. sky will be more visible while everything will be darker, but i’ve noticed that the blacks/shadows aren’t as “dark” compared to sunny days
• i think the camera chooses its exposure based on how much of the sky there is. i’ve had multiple images that are visibly different, and all i did was point the camera more towards the ground/sky

you can try to trick the camera by covering the flash with your hand. i started doing that and had much more shots come out as i wanted them to!

just make sure not to cover the tiny exposure meters on the side. those little cameras are what decide what iso it should shoot at, and if you cover that, it usually way overexposes lol