I don't know is the issue is Nokia 6.1 related or Android 8.1 related, but when one needs to write down some number during call, it needs to be saved to contacts before the call ends, because if not, the typed number will disappear immediately after call, and there's no way to restore it. This is so annoying for me as there are lots of times when I need to write down a number where to call next, but after the ongoing call ends, the number entered is lost and I have to call again to person who gave it to me and write the number down using pen and paper. Before Nokia 6.1 I never had this issue with any other phones, not with Samsung S4, and not with the old Nokia phones (3210 etc.). Please fix the issue or delegate the information on the issue forwards to someone who is responsible for the issue.
Another issue that is annoying is that sometimes when typing a message, sms, whatsapp or whatever, if the screen goes black while you are thinking what to write, the text entered has disappeared when you turn the screen on again. It is also really annoying when you lose a long text you have written because of it. Dunno if this is also related to Nokia 6.1 or is it application-related. But anyways, it's really annoying, and I don't remember having similar issues at all with my old Samsung S4.
PS. Please fix also the most annoying issue, the random hanging / freezing issue, that might be related to camera or not. Most of times it is the camera app that freezes, but it can happen anytime. Usually when turning screen back on and you have some application open.