It's hard to believe that the central government did not know or did not pay attention to what had been brewing in HK before the riot in 2019.
IIRC, there had been anti-establishment and anti-government protests on many 6.4 and 7.1 anniversaries. That's a clear sign of organized and coordinated covert operations. It's just impossible for the central government to have ignored all these. The authority actually had taken actions where and when it can. For example, Jimmy Lai had been sanctioned in mainland since late 90s. I have seen the retail stores of his apparel brand Giodano all closed down overnight in mainland.
Knowing what's going on is one thing. Having a solution is another. I believe the central government had, and still has, its hands tied up by 1C2S. It had been expecting the SAR government to take the initiative and had tried to provide support to the latter within 1C2S, such as the directives and law intepretations issued by the national congress.
For whatever reasons, CPC seemed to have determined that it would be a failure of 1C2S if the central governement intervened in HK openly, let alone by force.
