Since I updated the WordPress core and the theme around the same time, I am not 100% sure which is the cause, but my working theory is that something in the theme changed. What can I do to remedy this issue?
I have tried re-inserting the image as a full-size image, then scaling down to the proper size with no desired results. In the post edit page everything looks right, but once posted it's in a 150x150 square. I also tried using the Regenerate Thumbnails plugin which successfully ran, but had no impact on the issue.
Just to clarify, when I insert the images I am not inserting as a 150x150 image, but that is what it is showing up as. Additionally, posts that were made before the change have had the size of the images changed without the post ever being edited.
EDIT: Just as an addendum, from my relatively small sample size, it appears that only vertical images are affected. Horizontal images kept their original dimensions.