I had a support ticket opened for this a while ago (when I was running x2a) but it was closed as a system specific issue. There wasn't much else to do since I couldn't reproduce it. There are a number of posts about this under different names - time line, now time, now marker, etc. These appear to report the same thing - the Now Time Marker randomly disappears and will only come back after closing and reopening the project. Some have reported that doing something like toggling the inspector (I) will bring it back so that might be worth a try. My own personal observations were that it seemed to happen more with older projects (8.5.3), and when switching back from the PRV. I have X3e installed but I haven't spent enough time with it to say it continues for me.