Normally,LLR outputs when it can't prove a number prime:

Code:

Starting probable prime test of 2^21954+77899
2^21954+77899 is base 3-Strong Fermat PRP! Time : 1.728 sec.
Starting Lucas sequence
2^21954+77899 is Lucas PRP, Starting Frobenius test sequence
2^21954+77899 is Frobenius PRP! (P = 4, Q = 2, D = 8) Time : 6.928 sec.

But for 10^500+961 it doesn't run a Lucas or Frobenius sequence and instead says the number is not prime.

So I think it really is a bug.