According to this Google Issue Tracker post, comment #3.

These issues are all related to underlying data on Google Finance and are not specific to Sheets or the =GOOGLEFINANCE() formula.

The meaning of this is that it's due to a data issue on Google's side. It seems the issue with missing data for some stock tickers in Google Sheets, such as HKG:2800, isn't due to an error in your formula or Google Sheets itself. The problem is likely that Google Finance, which provides stock data to Sheets, doesn't have reliable or up-to-date information for those specific tickers. Therefore, even if your formula is correct and works for other stocks, if Google Finance lacks valid or current data for a particular ticker, the formula cannot return any results. Therefore, the problem isn’t with Google Sheets or the formula syntax.

This another post contains a suggestion that addresses this issue. It mentions that "Please use the Send Feedback button in Google Finance to report data issues. Please verify your ticker symbol. If Google Finance has the correct data, but =GOOGLEFINANCE() formula is failing, please send feedback directly within the Google Sheets Application menu. Help > Report a problem"