I'm doing what one should not do and file two simple bugs here. I'm not sure if they belong to Dolphin or the KDE interface. Please put them in correct group, if you don't mind.
I'm writing from a different system. Hence, I do not know the version, but as it's a Manjaro (amd64) system updated today (2020/06/15), you can assume that it's rather recent.

Bug 1: Cannot copy to ramdisk, KDE/Dolphin complains about it being full
My ramdisk is in fstab as:
ramfs	/mnt/ram	ramfs	defaults,mode=1777	0	0
When I drag a file to an open file lister in path /mnt/ram for copying, Dolphin will show a red ribbon claiming that the disk is full. Needless to say that doing the same via the command-line or any other tool works nicely.

Bug 2: Modifier keys as shortcuts for copying (CTRL) or moving (SHIFT) files between listers (Dolphin) only work when moving the mouse. On a desktop system this is unnoticeable, as the mouse almost always moves when doing this. On a notebook with a touchpad, however, this is annoying. One drags a file to another lister and presses SHIFT and nothing happens. Only when you move around on the trackpad, the arrow changes into move-mode.
Suggested solution (if it's implemented that way): Connect the key up and key down signals to the slot as well.
This seems to have been filed already in 2015:
https://bugs.kde.org/show_bug.cgi?id=352177
Unless I'm mistaken, they closed it?!?