The problem was eventually traced to a bug in HDR processing for photographs. Due to a software glitch, if a picture was taken and that the camera app was immediately exited -- before HDR processing could completely finish -- to perform another task on your phone, the photo would not save. For those that were hoping to capture a special moment or a once-in-a-lifetime event, this could be quite infuriating (to say the least).
Google has now responded that it is working to resolve the HDR processing bug, and provided the following statement to The Verge. “We will be rolling out a software update in the coming weeks to address the rare case of a photo not properly saving.”
The issue is specifically related to the Google Camera app and can affect the Pixel 3/Pixel 3 XL, Pixel 2/Pixel 2 XL, the Nexus 5X and even certain Samsung Galaxy smartphones. The issue apparently isn't affecting all of the above devices using the Google Camera app, but there were enough data points for Google to identify the problem and take the proper action.
Until the software update is pushed out, there are two easy ways to sidestep the issue. You can either 1) leave the Google Camera app open until HDR processing has completed for each picture that you take or 2) simply turn off the HDR feature completely.
You can read the HotHardware review of the Google Pixel 3 and Pixel 3 XL right here.