For some versions of software (viewer or editor), they may not fully support the exif specification, especially when that kind of software handle the MakerNote tag of exif. Since the MakerNote are manufacturer orientated, the detail information are reserved and may not follow the exif specification. When the image is modified, that kinds of software may store the information incorrectly. As a result the decoded information will be no meaning.
Known software with this problem:
Exif Viewer is only for displaying image exif information only and not plan to support the editing feature. It should be the responsibility of the device/software to edit the image exif itself to maintain the correct information.
If you have found any bug about Exif Viewer or if you have any good idea to improve the software, you can send to HMK409 with title starting "Exif Viewer - ..." and state your problem or suggestion in details.