So it's several things. Bear with me because finding news reports that I remember is difficult now because search results are flooded with stuff about the ban so I can't find what I'm looking for.
One concern is a general one that the Chinese government is directing the recommendation algorithms to act as propaganda. So subtly shifting user's opinions in favor of things that suit it and away from things that don't.
https://archive.is/tCVmR
Another is that it is using TikTok to surveil journalists, emigres, and other persons of interest who are using TikTok. My understanding is there are credible reports of journalists being targeted by the Chinese government, where they used TikTok to find their personal details, location, etc.
There's also been increasing reports of the Chinese government operating detention centers in the US and other countries, where they bring kidnapped Chinese nationals. Basically arresting nationals on foreign soil. In some of these cases at least TikTok has been implicated as the method of locating them etc.
https://theweek.com/speedreads/764194/intelligence-officials...
Discussion of this has all been out there over the years, but the way it's been covered has admittedly been weird. Maybe this is yet another sign of a fractured media landscape, but I think some of it has to do with the US not doing a great job of publicizing some things, possibly because it involves intelligence services.
I'm generally very in favor of unfettered freedom of speech, but have mixed feelings about this case. I guess I still side on that, and am skeptical about a ban, but this is getting into different territory and also don't feel strongly about it. I think the effects of foreign (and domestic) propaganda in social networks are very real, and although I generally think censorship is a very bad idea, I'm not sure I can blame a country for wanting an app banned if there's solid information that another country is using it in this way; it seems to be in this gray area of espionage versus free speech which is kind of an unusual territory to be in. Also, I'm fully aware that the US probably does similar things, but two wrongs don't really make a right to me, and if China produced solid evidence of the US doing something similar I wouldn't blame them for banning something either on similar grounds.
To me this all just maybe speaks to the need for a shift to open decentralized social network platforms. I realize that's easier said than done, but there's so many examples in the last few years of problems with control of centralized platforms (by private, government, or private-government combinations) leading to huge problems, either in reality or in appearance (which can sometimes be almost as equally concerning).