After Unpacking the zip Files of my Flickr Data Export, I found the Metadata in json files and Photos without the Original Modification Date.

Using exiftool on mac I managed to Re-Import the EXIF Data and Adjust the Date to the Date of the Photo taken

1. Install exiftool command
https://sourceforge.net/projects/exiftool

2. adjust the folder path in the dir variable containing your unpacked flickr export data

3. Run This One Line Command in Terminal and it will pick the matching id from the json file and apply to the related photo:

dir="/Users/islam/Downloads/my_flickr_export"; for json in $(find "${dir}" -name "*.json" -type f); do id="$(basename "${json}" | cut -d. -f1 | cut -d_ -f2)"; photo="$(find "${dir}" -name "*${id}*" -not -name "*.json" -type f)"; exiftool -m -d "%Y-%m-%d %H:%M:%S" -tagsfromfile "${json}" "-Keywords<tags" "-Subject<tags" "-Caption-Abstract<name" "-ImageDescription<description" "-DateTimeOriginal<date_taken" -ext "*" -overwrite_original -progress "-filemodifydate<date_taken" "-filecreatedate<date_taken" --ext json "${photo}"; done

Finally, you can delete json Files

dir="/Users/islam/Downloads/my_flickr_export"; find  "${dir}" -name "*.json" -type f -delete

I have similar working solution for Google Photos takeout too.