Is elliptic curve point multiplication semantically secure?
If an attacker can choose the points $P_i$, than this system is not semantically secure. For example, they may choose $P_2=2P_1$, and the corresponding encryption $Q_2$ would be equal to $2Q_1$.

If the points are chosen at random, this system is semantically secure if decisional Diffie-Hellman assumption holds for the curve. This assumption is presumed to hold for elliptic curves typically used in cryptography, unless the curve is chosen to have a bilinear pairing.