Indeed, I can confirm that this is a mistake I did.
Using #if UNITY_2018_3_OR_NEWER && UNITY_EDITOR is the correct fix.
The CutsceneGroup.cs unfo related calls, are already wrapped in UNITY_EDITOR by the way (all Add and Delete methods are in fact).
I have also attached for you here a fix related to build regarding rotations bug in UnityObjectUtility.cs for your convenience.
I will update the package shorty with the fix.
Thank you! 🙂