Yes. The rule of law.This is a part of "one country, two system". The solution is issuing a lot of clear laws to guide their behaviors.
Basically the Judiciary System of Hong Kong SAR remains the same as pre 1997 under the former colonialist with new teeth added e.g. National Security Laws as permitted under the Basic Laws.
That is why British Ministers were caught dumbfounded when asked on which part of the Basic Law of the One Country Two System was breached. They became defensive and evasive because they could not answered. It is truly within the context of the signed agreement.
In China like in Korea, Taiwan, Japan and France, one is presumed guilty unless investigation prove otherwise. So one remain in jail until the investigation is finished.
But in Hong Kong it is still innocent unless proven guilty hence the process may take a bit longer.
All thise being charged in Hong Kong are currently presented to court after a lengthy investigations by Hong Kong Police.
That is why they are only arrested and charged today. In USA the arsonist are considered as terrorists.
Most of them are not charged under the new National Security Laws as reported and alleged by Western News Media but rather under the existing British Common Law.
Hong Kong is the 2nd last card that the hegemonists can played and it will be checkmate soon for them.
Same applied to the final card Taiwan. Checkmate when reunification is completed.