GIMP doesn't properly remove image data when an alpha channel is supported
When you save the image to a format that supports alpha (e.g. PNG), any "removed" picture information isn't actually removed. GIMP simply adjusts the alpha channel to make a hole through the picture. From a UI perspective, the above picture sure seems that it has content removed. But in actuality, anybody can simply remove the alpha channel (e.g. with ImageMagick "convert" or other tools) and they will have the original, unredacted photo.
When you save the image to a format that does not support alpha, but does support a thumbnail, that thumbnail also contains the unredacted picture. Possibly GIMP is removing the picture's alpha channel to generate the image for the thumbnail? This is clear when using a tool that leverages an image's thumbnail, e.g. the default Ubuntu file manager:
Both behaviors are contrary to what a user expects. And as such, they may be unexpectedly sharing information in photos that they believe are properly redacted.