Does not happen with a clean config.
I just shared my config with Mike, so you can get a copy from him if interested.
Help-> About shows only 2 monitors.
I have 2 externals, plus the laptop monitor.
2 2560x1440
1 1600x900.
SlickEdit Pro 2019 (v24.0.0.4 64-bit)
Serial number: FE33710_BETA
License type: Beta License
License expiration: 2019-10-27 17:00:00
License file: C:\ProgramData\slickedit\24\slickedit.lic
Build Date: September 4, 2019
Emulation: Visual C++ 6
OS: Windows 10 x64
OS Version: 10.00.0
Memory: 57% Load, 9395MB/16222MB Physical, 10299MB/17222MB Page File, 4747MB/134217727MB Virtual
Shell Information: C:\WINDOWS\system32\cmd.exe /q
Screen Size: 2560 x 1400, 2560 x 1440
Project Type: (Other)
Language: .cs (C#)
Encoding: Automatic
Installation Directory: C:\Program Files\SlickEdit Pro 24.0.0 Beta3\ (non-removable drive,NTFS,83880MB free)
Configuration Directory: C:\Users\jporkka\Documents\Slickedit3\24.0.0\ (non-removable drive,NTFS,83880MB free)
Spill File: C:\Users\jporkka\AppData\Local\Temp\$slk.18700 (non-removable drive,NTFS,83880MB free)