Exponentiation Problem of G2 in MNT curve
Here's a guess: when you ask to compute h ** (a * b), it actually computes h ** (a * b mod q), where $q$ is the order of $g$ in $G1$.
If that is the case, well, a random element from $G2$ is not going to be a divisor of $q$ - any element that is the result of a pairing operation will be; however most elements of $G2$ cannot be the result of a pairing operation.
And, if the order of $h$ is not a divisor of $q$, then $h^{a \cdot b} \ne h^{a \cdot b \bmod q}$
The above would give the expected result if you set $h$ to be the result of a pairing operation, possibly with two random $G1$ elements...
(This is labeled a guess because I'm not familiar with the Charm framework, so I'm guessing about its behavior)