I consider this a defect, but at a minimum it is serious behavior that people need to know about. I use Photoshop Elements 15 Organizer to catalog and manage my collection of 60,000+ images. I also use Photoshop CC 2017 and Adobe Camera Raw to enhance my images. I just went through a lengthy process of scanning about 200 old 35 mm slides to .tiff files. Once scanned I made first pass adjustments with Adobe Camera Raw, then did fine color adjustments and spot removal with Photoshop CC 2017. At that point I imported the adjusted images into Photoshop Elements15 Organizer and added captions, date information, and tags. When viewing these images in PSE Organizer I noticed a few of them needed some additional adjustment, tweeks that I missed on their initial pass through the enhancement workflow. So, I opened these files one at a time in the Adobe Camera Raw - Photoshop CC 2017 sequence and corrected the images, then stored them, replacing the original files. When I went back to the PSE Organizer I used the "Update Thumbnail" operation to display the corrected images. The corrected images were displayed, but PSE Organizer also went into the files' meta data and extracted some contents replacing the "Caption", "Tags" and the "Date and Time" information with whatever existed in the files' meta data fields. As a result I lost all of the information that I had painstakingly created for captions, tags, and actual shot dates.
It appears that the information I lost had only been stored in PSE Organizer's local database. What will avoid this problem is to ensure that the "Save meta data to file" operation is run on each image after new meta data is entered into the Organizer. Doing so causes the "Caption", "Tags", and "Data and Time", and possibly some other information to be written over the meta data fields in the image, thus preserving that information and avoiding the loss of it from the PSE Organizer's data base if an "Update Thumbnail" operation is performed.
I actually consider this a serious defect in the product, because the results of doing an "Update thumbnail" operation does not only update the thumbnail (a visual representation of the image) but updates other meta data for the image which is extracted from the file's meta data and over wrights what was in PSE Organizer's database. The operation does a lot more than it advertises and thus can lead to serious adverse consequences for the user.