Josef Stalin II, the heavy tank, encountering Tiger I

Tiger I

IS-II

The first known operation which saw the IS-IIs heavy tanks took place in Romania in April 1944 were the newly formed 11th Separate Guards Heavy tank Regiments fought several skirmishes against the s.Pz.Abt. 503. A Soviet tank was knocked out but the Germans could only take a brief look at it before being forced to retreat. A month later, again in Romania, near the town of Tirgu Frumos, another engagement along the line of the Soviet strategic trust in the Southern front took place. Tigers from the Gross Deautschland Panzergrenadierdivision engaged the IS-2s on long ranges. 

General Hasso von Manteuffel, commander of the German Korps in the area, said: "It was a surprise discovering that, while my Tigers started to hit them from a 2,200 metres range, our projectiles were not able to pierce them until we got to about half of such a distance. But I was able to counter their technical superiority by manoeuvring to use in the best possible way the ground cover." 

He intended that the Russians failed to hit small targets over the 1,200 metres: this was undoubtedly due to the IS' poor aiming system and the usual difficulties to lock the target on long ranges. But it was certainly a bad surprise for the Tigers' crews (and for that handful of Tiger IIs just entered in service) to receive a 122mm 56 pounds projectiles fired by an IS-2. On 2,000 metres the 122mm was able to pierce the Tiger's I front armor. The Germans could not fire anymore while standing up on their tanks.

However, it is said that it was never proved that any Tiger II was destroyed from a hit in front. From lateral and from back they were many times. 

Mostly of above was picked from a site about Iosef Stalin II tank. As you can read in text, there is a mention about few Tiger II being around. Personally, I don't endorse the idea that Tigers II were involved in battles for Tirgu-Frumos. As far as I know, the Tiger II prototype was tested near Kiev in May 1944, then many more were involved again in Poland in August 1944.