Why are my images recompressed?

There are situations when the images that were previously compressed appear as unoptimized and they are processed again by ShortPixel Image Optimizer. This happens when our image optimization data on WordPress is removed, and the plugin "believes" that the images aren't compressed.

Causes

Note: Each website is a different world, and the following reasons may not include your specific case.

Some of the possible causes for the missing data are:

Other plugins altered/deleted the image optimization information from the WordPress database. The most common suspects are the thumbnail regeneration plugins, thumbnail editor plugins, watermarking plugins, or any other plugin that edits the images in some way.
The site was moved to another hosting/domain without copying the image optimization data located in the WordPress database.
The images were renamed using a destructive method. There are file rename plugins that trigger the re-optimization because they make ShortPixel "believe" the images are new.
The "Remove all ShortPixel Data" button located in Settings > ShortPixel > Tools was clicked. This action triggers the removal of the image optimization data from the WordPress database. Even though your images will remain optimized, the next time the plugin is activated, it will no longer recognize previous optimizations.

WooCommerce: WooCommerce has an integrated thumbnail regeneration tool that works in the background. If you use WooCommerce, it is very likely that WooCommerce triggered the image reoptimization.

What to do with your files?

If your Media Library still has unoptimized images, you could reoptimize those unoptimized images with the Lossless method. This way, the WordPress metadata would be restored and the plugin would see again the images as optimized. Once that is done, set again your optimization method to Lossy.
If your Media Library has been reoptimized twice or more, and the quality of your images has not changed, you can leave your media files as they are currently.
If your Media Library has been reoptimized twice or more, and the quality has degraded, you should restore the images to their original state (Settings > ShortPixel > Tools > Bulk Restore).

How to prevent this issue from happening again?

If you have a thumbnail regeneration plugin, we recommend replacing it with reGenerate Thumbnails Advanced, which is well integrated with ShortPixel.
If you use WooCommerce, disable its thumbnail regeneration feature. For more information, click here.
Always make full backups of your website, including the database.
Do not rename your files with a plugin. Use Enable Media Replace instead.
If you have any other plugin that edits your image files in any way, keep an eye on it as it may also be the cause of this issue.