Thanks, Tim C.!
Un-docking and re-docking the Object Info Palette did fix the problem.
Besides, I notice that we can now update to Vectorworks 2019 (SP 3.1) and it permanently fixed it without un-docking and re-docking the palette.
I've struggled with this problem and have to use Vectorworks 2018 instead for months. Hopefully, the new update will be fully compatible with MacOS Mojave.