At the time of writing, the introduction of Maneuvre Mode in KSP 1.6 broke the detection of the action set to activate in game. See bug https://bugs.kerbalspaceprogram.com/issues/22165 for more details.
Thanks to the well documented public class "SteamController", writing a plugin that will activate Action Sets is not difficult. So, I reimplemented mine... even with some "help".
To use this plugin :
- Delete the Squad Plugin : /GameData/Squad/Plugins/KSPSteamCtrlr.dll
- Add the following plugin in /GameData/SteamControllerPlugin/SteamControllerPlugin.dll
Et voilà !