There is a urldecode on the $file_name which is used as a the uri property when trying to load the file. The problem is this decodes urls like 'my+file.jpeg' into 'my file.jpeg' and doesn't match the stored uri property and therefore doesn't match the file. See https://www.php.net/manual/en/function.urldecode.php where it says "Plus symbols ('+') are decoded to a space character."
Add a media item with a file name containing a plus symbol in it. Then try to generate an image derivative for it using drimage_improved and note that the file can't be found and no image is produced. It will throw a log message "Image not found"
Remove the urldecode from line 139.
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Now I've found a situation where the image is only loaded if there is a urldecode(). So maybe urldecode is needed but + symbols should be ignored somehow.
This edit makes use of rawurldecode() which leaves the plus symbol alone and does this: "Returns a string in which the sequences with percent (%) signs followed by two hex digits have been replaced with literal characters."