When multiple files are loaded at one time when the Application Frame is disabled it will result in an annoying screen flicker/blinking of each file loading as a new window. The more files that are loaded at once the more blinking you will be blinded by.

Usually of Illustrator preloads all of the files first and then loaded the main window/set of tabs, as apposed to loading each tab one at a time with this visual annoyance.

I've tested this on earlier on Illustrator 23.1.1 on High Sierra and this issue does not occur.

It does occur on my primary iMac running Mojave.

My Setup:
iMac (Retina 5K, 27-inch, Late 2015)
4 GHz Intel Core i7
Memory 16 GB
AMD Radeon R9 M395 2GB

Resolution: 5120 x 2880
Mac OS 10.14.4

I am installing Catalina now to confirm if the issue occurs along with other more extreme white flickering issues.