Best I can tell is that the metering system didn't get the exposure right in the second shot. From EXIF data, the camera was set to auto-area AF mode. In this mode, the camera selects the location of the subject within the frame. The scenes' selected subject (e.g., its color) is part of the matrix metering algorithm. For the first shot, the camera selected a point directly on the large tree to the left of the person. The camera's primary focus point was on this tree and at about the person's knee level. For the second shot, the camera selected a point between the person and the same tree at a point just beside her feet as the primary focus point. That point was quite a bit lighter than the tree and, perhaps, contributed to the underexposure of the second image. Also, another change between the scenes is that, the person (or camera perspective) moved to the right leaving a larger area between the tree and the person in the second shot as compared to the first. With the second shot using this brighter area between that person and the tree as the subject, this likely also contributed to a difference in metering. In fact, while most of the second shot is dark, the subject area between the person and the tree is not terribly underexposed. I believe that you would have had more consistent exposure results if you had not let the camera select the subject focus point. Instead, a single point AF mode (manually set over the person's face) is probably a better approach.

Yes, sometimes auto iso doesn't work great. The cool thing I've noticed about auto iso is the camera can choose increments that are not available to me when I set it manually. When I set it manually, mine starts at 100 and each setting after that is double, like 200, 400, 800 and the like. What I've noticed about auto is that the camera has the liberty to choose a "just right" setting like 160 or 240 or 320 etc. My P900 is wonderfully judicious about iso keeping it surprisingly low in a variety of situations. My Canon T5i not so much. Seems to be much more light hungry. It makes me wonder if I missed that part of the owner's manual that allows me to dial in non standard iso values. I agree with Steve, your incident seems like a freak occurrence.

Ha! And no it was the jet stream taking an abnormal northerly spike that allowed the high pressure system to penetrate so far north. We suffered along with you! However, I'm glad that you guys got to Muir Woods, it is a great location, but I always find it challenging lighting wise. It's actually one of the reasons that I seldom set my camera to auto ISO, or matrix metering, especially in challenging lighting environments with widely varying highlights.

Looking at the EXIF data you had consistent shutter and aperture settings, but the ISO went from 5600 in the first image to 1100 in the second. My suspicion is that even a slight shift in framing could confuse the matrix metering algorithms. I would doubt that your camera has any faults, just that the lighting environment was simply too complicated for the matrix metering system. Looking carefully at both images, the highlights behind your subject are in slightly different positions, which is probably enough to alter the results from the metering algorithms.

Your example - once I was the right way up, I was horizontal when I first read it - made me think about my old Canon T90 which had a multi-point spot metering system, which would have been very useful in a situation like this, I wonder why that's no longer being used on more modern cameras. I didn't use it often, but in situations like this it did an excellent job of finding a good exposure value.

Anyhow, I hope you guys had a great vist to the Bay Area, despite the temperatures.

Take care!