Because of the effects of Shi Yiling's mask, Qian Cangyi temporarily forgot certain matters. However, it's worth mentioning that the period of memory loss spans from the moment he entered the movie world to the time the mask was used. After using the mask, his memory remained intact. In other words, the memory of Jia Nian stabbing the Rainbow Short Sword into him for the first time, up to the moment of Qian Cangyi's death, was unaffected. Moreover, the memory from his "resurrection" to the present was untouched, while all other memories have vanished. Yet, even with just this section of his memory, he could deduce a significant amount of information.
Firstly, Shi Yiling's mask had already been used. Considering that this special prop can only target its user, it is evident that he himself had deployed it. Secondly, the duration of memory loss determined the moment the mask was used, which helped him comprehend his current situation.
