I have just installed AutoCad 2015 to test it out and have come across this problem. I drew a simple box with a hole in it in the WCS. I rotated the UCS along the X axis by 90 degress. Started the line command, snap to the corner of the box (3D snap vertex is running) and the "dynamic line preview" is locked to the WCS until I click an endpoint - then the line is drawing correctly in the UCS, but the "next continuing" line is still locked on the WCS until I click another endpoint. I'm really hoping this is just a simple setting I have missed somewhere, otherwise I will have to uninstall and go back to 2014 as we do all of our drawings in 3D. I have attached a video to show what is happening.
(I had to zip the video to get it to attach)
Ok got it working. Check your dynmode system variable. Mine was set to -3, which is apparently not a valid option. Valid options are: 0,1,2, or 3. I suspect it may have been because I migrated my settings from a previous version? Who knows but I'm happy that it works again.
So, 2015 has an issue with drawing the active line segment (while using the LINE) command, but only under the following conditions:
1) UCS is something other than WCS, and
2) DYNMODE is set to 0 or 1.
So, you have to live with all the distracting dynamic input if you are drawing in a UCS other than WCS. In other words, set DYNMODE to 2 or 3 to cure the problem.
Hope this helps. I think AutoCAD may have finally figured this out by 2019 version. It took me an hour to figure this out. SMDH.