iPhone version 5.6 has been released. Version 5.5 was released in December 2017. Both these releases fix various bugs that had become evident. All these bugs were in the user interface and did not affect the computation results.
None of these bugs affected the Microsoft Windows version nor the Apple MacOS version. No version 5.5 nor 5.6 has been released for these platforms. Nevertheless, the next release for these platforms will be named 5.6 or later to keep the version numbers consistent between the various platforms.