It is not possible to use the "Server/AwardSteamAchievement" API call multiple times. Once an achievement is triggered using this call, it can no longer be triggered, even when resetting the Steam Achievements.
When an Achievement-ID does not exist on Steam yet when this API call is used for the first time using the ID, the achievement can't be triggered at all (since it is already cached internally so it never reaches Steam)
After contacting the support, I found out that this is working as intended - even though this internal caching is not described on the documentation Page for "Server/AwardSteamAchievement".
It would be helpful, if there was either a way to reset this internal caching or a way to bypass it. Not being able to test achievements multiple times on an account makes this feature unusable in any QA/testing process. In any way, this should be documented as it was impossible to debug this feature, since the call does not return any error for this. (only a "success: false")
