multiple things could be the reason, I can only speculate:
Depending on the length of the input there might be different heuristics of what to do with it. e.g. if it is short, it is more probably to be (say) an animal name, then when it is a very long input. Perhaps a similar construct is in place for equations.
Also, there might be a construct in place that makes sure that (very long) input does not crash or unnecessarily overloads the system.
Perhaps you can run it in the Wolfram Cloud or (if available) on your own computer using Mathematica.
