I've had this issue for some time (since before 4.0) and it has never gone away. I'm not sure if it is a problem between BC and my system or if this problem shows up with other users. I'm forced to work with pdf vector data a lot and joining lines is a necessity. However, when joining lines after a while an error occurs in my viewport that causes the background to become very pale, the lines to be come very bright and thick. If left unchecked, it usually occurs again which causes a complete loss of line control then usually followed by an error message stating I need to save BC and restart the app. Starting a new plan view window and closing the old one usually fixes the issue, but recently it has been skipping the message and locking up BC without me being able to save. A few times I get a Windows error message stating "Program has encountered an error and needs to close..." All while joining lines. I have noticed it happens most often when joining 3d contour lines to 2d contours. If I get that error message again I will post the contents.
I typically would not complain but it seems to be getting worse. Hopefully someone has seen this before and can let me know a fix. I've attached photos below of before-after. Thanks!
Normal lines before error. Pink lines are zero elevation; green have been raised to grade
Normal pdf and linework in viewport
Note red crosses on line grab points. At this point lines cannot be moved by grabbing
Lines are thicker and brighter. PDF has faded. Green line on right is boundary of pdf image that shows up after error