This problem existed in TC 2017 Deluxe as well, but is much more frequent in TC 2018.
The problem is related to selecting objects following 3D Boolean operations. It goes like this: I do a 3D Boolean operation, doesn’t matter which, and when attempting to do a follow on 3d Boolean operation I will often be unable to select the new just created composite object. Alternately I might be able to select the first object for the operation but not the second one.
I have tried switching from the model space to the paper space and then back, but that never corrects the problem. The only thing that works is save the file, restart TC and then reopen the file, which works every time.
I’ve been using TC for about a year and have done thousands of these operations. Originally this only occasionally occurred, but currently I have this problem 1 out 4 Boolean operations and it costs me a lot of wasted time.
This occurs regardless of rendering mode and regardless of which rendering engine is used.