Monobehaviour resets variables after the initialization cycle

I have a really weird problem. I've written a code that looks like this:

 UtilitiesList utilities; // Not derived from MonoBehaviour
 bool DEBUG_UtilitiesExists = false;
 
 void Start () {
 utilities = new UtilitiesList ();
 
 DEBUG_UtilitiesExists = utilities != null;
 Debug.Log ("DEBUG_Utilities exists: " + DEBUG_UtilitiesExists);
 
 //... Here I create utilities and add them to the UtilitiesList
 }
 
 void FixedUpdate () {
 
 if(utilities == null)
     Debug.Log ("DEBUG_Utilities exists: " + DEBUG_UtilitiesExists.ToString ());
 
 utilities.Update ();
 
 //... Further processing
 }

It works properly in 99 % of cases. But sometimes when I start the game it throws a NullRefernceException in the following string from FixedUpdate():

 utilities.Update ();

The most astonishing is that no exception is thrown from the Start() function! So initialization seems to be performed properly. And this is the only place where I assign a value to the "utilities" variable (and this variable is private). That makes me think that it doesn't depend on the script execution order; anyway, dependencies from the other monobehaviour components are not tight.

I added some variables for debugging as you can see above. And when it occures that the "utilities" variable becomes a null reference in FixedUpdate, "DEBUG_UtilitiesExists" variable is always true. That means that the initialization in Start is always performed successfully, but somehow it resets the reference somewhere between Start() and FixedUpdate().

The same situation occures with the other class which works with the XML files. The variable that is reset on its own, is a public property with a private set method.

I have absolutely no idea, what's happening)) Only that this is a Unity bug. If it is so, will it occur in the built game?