On 05/05/2021 we re-imported the Stronghold: Warlords achievements to the GOG servers. Unfortunately this caused all GOG achievements that players had completed before 05/05/2021 to be reset. As soon as we were aware of the issue we contacted GOG to see if they could revert the changes but sadly they could not. We apologise for this unforeseen error and promise to not make the mistake again.
If you would like GOG to manually complete your missing achievements, please contact their support (https://support.gog.com) with a list of your achievements.