GetAppInstallDir() always null-out and SteamApps.GetDLCCount() ignore soundtrack DLC

[Bicardine]

So I use

      var dlcCount = SteamApps.GetDLCCount();

            for (var i = 0; i < dlcCount; i++)
            {
                SteamApps.BGetDLCDataByIndex(i, out var appId, out var available, out var dlcName, 128);
                var isInstalled = SteamApps.BIsDlcInstalled(appId);
                ...
             }

to get dlc data (with dlcAppId) to show DLC UI in-game and then
var folderNameLength = SteamApps.GetAppInstallDir(new AppId_t(dlcAppId), out dlcFolderPath, BufferSize);
But dlc folder path is allways null for any purchased DLC (game is published as well as DLC is installed on user PC or not any way).

SteamApps.GetDLCCount() also return all dlc count, but ignore soundtrack DLC for some reason