Two Dolphin instances, each open to folders on different drives, Detail view. Select 40,000 files in one folder, drag/drop to the second folder, choose Move. As each file is transferred, it appears selected in the new folder. The status line on the destination folder shows the number of selected files increasing as the number decreases in the source folder.

If you change directory of the source folder so those selected files no longer appear, the remainder of the destination folder's files are not selected, the destination number of selected files remains static, displaying the number of selected files before the source folder changed. The selection status of the source files follows it over to the destination.
However, if you then go back to the source folder and re-select the source files, the destination folder's selected files remain as they were. This suggests that only the initial selected files are moved to the destination folder carrying their selection status with them. 

This experiment was performed while moving 46,000 images from one drive to another, a 4+ hr process while another dolphin instance was moving 75,000 files to a different folder at the same time.
[Typical file numbers for post-production video work. All the image frames of the video are extracted, altered via a Gimp script, then re-assembled into a new video. A 40Gb video produces over 3Tb of frames.]

EXPECTED RESULT
The destination files should be un-selected until they are explicitly selected in the new folder. 

Operating System: Slackware 15.0
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.89.0
Qt Version: 5.15.3
Kernel Version: 5.15.11 (64-bit)
Graphics Platform: X11
Processors: 16 × AMD Ryzen 7 1700 Eight-Core Processor
Memory: 15.6 GiB of RAM
Graphics Processor: Radeon RX550/550 Series