My concerns are largely over the precedent that might allow banning end-to-end encrypted apps over "national security," but there is an argument to be made on 1st Amendment grounds.
There are two 1A considerations here: whether an "app" is speech, and the speech of TikTok's users. If courts decide that TikTok is a "public forum," then there would be precedence to prevent the privation of this public forum in the interests of free speech. [1] On the other consideration, one of the landmark cases regarding software, Bernstein v. Department of Justice, ruled that code is speech, and this directly lead to the end of export control on encryption. [2] This has been further affirmed by a ruling in favor of the video game industry against a law banning the sale of games to minors in CA. [3]
[1] https://www.law.cornell.edu/supremecourt/text/482/569
[2] https://www.eff.org/press/archives/2008/04/21-40
[3] https://www.law.cornell.edu/supct/html/08-1448.ZS.html