Looking into this, the amount owned by a single foreign entity is only 10% and any Chinese company can have up to 30% of its stock owned by foreign investors.  This source is one example for the Shanghai Stock Exchange, but I found many sites quoting the same for Chinese stocks. Therefore, if SPX were to list, POET would need to make a substantial sale of their stake.

https://english.sse.com.cn/access/qfiirqfii/rules/ 

• Shareholding by a foreign investor in a single listed company shall not exceed 10% of the total number of shares of the listed company
• Aggregate shareholding of A shares by all foreign investors in a single listed company shall not exceed 30% of the total number of shares of the listed company