Android Firebase transaction having a simple counter problem

Question

I've a very simple countdown in my Firebase database

I have this rule granting the node's access:

"actions":{
         ".indexOn":[
            "timestamp"
         ],
         ".read":"auth != null",
         "$act":{
            "countdown":{
               ".write":"auth != null && data.val() - newData.val() == 1 && newData.val() >= 0"
            }
         }
      }

the innitial value is only set manually by me... so at runtime users only can read and decrement value by 1. there is absolutelly only one call to write this node in my code and I use the following transaction:

@NonNull
public Transaction.Result doTransaction(@NonNull MutableData mutableData) {
    long counter;
    if (mutableData.getValue() == null)
        counter = this.action.getCountdown();
    else
        counter = (long) mutableData.getValue();

    if (counter > 0) {
        mutableData.setValue(counter - 1);
        return Transaction.success(mutableData);
    } else
        return Transaction.abort();
}

public void onComplete(@Nullable DatabaseError databaseError, boolean commited, @Nullable DataSnapshot dataSnapshot) {
        if (databaseError != null && !databaseError.getMessage().contains("network disconnect")) {
            Crashlytics.log("firebase user: " + FirebaseAuth.getInstance().getUid() + " error on action: " + this.action.toString() + " " + (dataSnapshot != null ? dataSnapshot.getValue().toString() : ""));
            Crashlytics.logException(databaseError.toException());
        } else if (commited) {
     do stuff
        }}

but I'm getting a very high number of "Permition Denied" during my onComplete. all cases the: FirebaseAuth.getInstance().getUid() is != null. so I'm sure there isnt any authentication problem.

My app is used by around 5k users per day
what is wrong with my code to explain why so many Permition Denied in such a simple counter?

======= UPDATE ==========
Explaining the context of this: this code runs only during the 'onCreate' of the Activity called right after login.
The innitial if should not even be necessary since im 100% sure user only gets here after a sucessful login, but as I didnt find anywhere a concrete oficial information saying if the FirebaseAuth can be invalidated when the user switches between apps for a long time I found easier just place the if.

this is absolutelly the only code that touches the refered database node.

if(FirebaseAuth.getInstance().getCurrentUser() != null) 
    App.getDatabaseInstance().getReference(Firebase.BRANCH_ACTIONS).addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull final DataSnapshot dataSnapshot) {        
        if (dataSnapshot.hasChildren()) {
            for (final DataSnapshot actionDataSnapshot : dataSnapshot.getChildren()) {
                try {
                    final Action action = actionDataSnapshot.getValue(Action.class);
                    //noinspection ConstantConditions
                    max = action.getTimestamp();

                    if (action.getCountdown() > 0)
                        actionDataSnapshot.child("countdown").getRef().runTransaction(new ActionTransaction(action));
                } catch(Exception e){}
            }
        }
    }
});