Well, I finally found the culprit: Acronis True Image 2018. Apparently, when you install Acronis True Image, it also installs an anti-ransomware product called Active Protection without telling you. Nor does Active Protection provide you with any notifications that it has blocked anything else from running. I went through and examined every single running process and application on my system. I skipped over Acronis multiple times because it's supposed to just be backup software. And I had already verified repeatedly that Windows Defender firewall and anti-virus weren't blocking it.
It wasn't until I actually went through everything the third time and finally looked at Acronis that I found the screen that listed Tower!3D's running processes as suspicious that I figured it out. Once I added tower3d.exe, mmwindow.exe and towerspeakbridge.exe as exceptions the 2nd monitor popups started working.