Nothing is absolute in biology. Your edge case can happen, but the chance it happens is so low that it might as well be your body naturally created the tumor in the first place.

As someone who handle cells on a daily basis, I can confidently say these things are the most fragile things ever. You look at them wrong and they will be hurt, literally. What's that? Somebody forgot to feed me today? Welp, time to die. Oh wait, you actually fed me a day earlier? Well, dying time I guess. Hmm, you fed me on the right day but the feed was not warmed up? You guess it, straight to the grave.

For them to be frozen, put in a plastic bag, thawed out, left out for an hour, then exposed to your immune system who gonna be overzealous in killing them and asking them to survive and grow into a tumor is like asking is there a chance a cosmic ray gonna flip a bit in my hard drive and completely corrupt all my data.