My iPhone 5s has a bug in the free 15c app. If I enter 6.5 and then convert it to HMS I get 6.3. If I then convert that back to H I get 6.5111. Has anybody else seen this? There is no entry in the phone settings menu for a 15c app, and my iOS version is 11.3.
I beleive you have a non-official version of the application. There are/were a lot of fakes out there and many of them did not work well at all. If you don't have a "settings" in your normal settings location, that defitinitely is not the real application as I'm nearly certain that has been there since day 1 of the initial version we ever uploaded.
Also, there is no "free 15C" app produced by HP.
Although I work for the HP calculator group as a head developer of the HP Prime, the views and opinions I post here are my own.