Why does my iPhone 15 Pro Max's camera only allows me to focus on the moon in video mode but not with photo?

Your photo has already the perfect exposure time, but is a a bit blurry. The 5x zoom of the iPhone 15 does not suffice to take a portrait of the Moon. Avoid zooming in further than 5x, or the photo may become blurry.

And avoid that the iPhone will use Night Mode or Deep Fusion with a long exposure time. This will just add motion blur. The Moon is so bright, you need a very short exposure time.

iPhone 15 Pro Max camera unable to focus … - Apple Community

Have you tried to take the photos as RAW photos to avoid the Deep Fusion or Night Mode? And use Exposure Compensation Control to avoid over exposure:

Just tap the arrow , then tap  and adjust your exposure level. The exposure locks until the next time you open the Camera app.

I tested yesterday during the super moon:

My iPhone 15 pro Max kept switching from the 5x tele lens to the optical zoom, when I Photo mode and made the image blurry in night mode. It did not happen, when I reduced the exposure time and set the camera to shoot RAW and the exposure time has been well below 1 second.

The photo below is a heic image, taken in night mode, hand held camera: It is still slightly blurry.

Data from Metapho: