My iPhone 5s is having same problem and after writing to the Apple community I came up with the solution that my Phone "Digitizer" may be having problem and needs replacement.
Furthermore, my phone was behaving normally since iOS 10, but after upgrading it to the latest version now, I'm !#^&@@ off whenever I'm trying to type text on keyboard or sometimes now even if the phone is in standby mode.
I guess now its time to change the device, as it's in my hands since 2014.