I know this thread is very old but I was just experiencing the same issue. In my case, I am using Nginx as a reverse proxy and managing SSL there. If I bypass Nginx and connect directly to the Ignition gateway, this issue goes away.
I had a similar issue with Designer Launcher however the details here seem to be different.
The request that is failing is to /data/perspective/runnable-projects. If I make that same request from another client like a browser or curl, the request succeeds through Nginx. From the mobile client (iOS), Nginx responds with a 400 status. It appears that the mobile client is making an unusual request that the Igntion gateway can accept but other servers like Nginx will not.
The same 400 response is given to the mobile client when it verifies the hostname and port by requesting /system/gwinfo however the mobile client appears to be happy with any response and verifies the address even after receiving a 400 status.
In the case of using Nginx, I do not see a way around this. Hopefully one of the product owners can weigh in.