I'm using a Debian 10 installation in which some parts have been upgraded to Debian 11.
KDE Plasma version is 5.26.90, Frameworks 5.102.0, Qt 5.15.8, Kernel 5.10.0-20.

When I open nedit (with a Debian version number of 1:5.7-3) from an XTerm command line, it opens in a different random workspace, usually workspace 4 (out of 6), but sometimes workspace 2. Then I have to go hunting for it.

This started happening after I did "apt-get update; apt-get upgrade" on the advice of a correspondent. Probably a bad idea to have done it, but I can't undo it. At the same time, I cannot now open it from an icon on the toolbar. It silently does nothing. I tried changing the icon properties to use kstart5 with the --currentdesktop option, but it still doesn't open. If I open it from the command line using kstart5 with the --currentdesktop option, it still opens in a different workspace. If I go to the workspace where it opened, and open another one, the new one opens is yet another workspace. Never the current one.

Is there a plasma setting somewhere that might affect this?

No other application does this.

Follow up: With gnome, nedit opens in the same workspace from which it is launched, either from a terminal or an icon. Even using a 2004-compiled antique 32-bit nedit opens in a random workspace, not the workspace from which it was launched. It's not a nedit thing. It's not an X thing. It's a KDE thing.

Operating System: Debian GNU/Linux
KDE Plasma Version: 5.26.90
KDE Frameworks Version: 5.102.0
Qt Version: 5.15.8
Kernel Version: 5.10.0-20-amd64 (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i3 CPU 530 @ 2.93GHz
Memory: 3.7 GiB of RAM
Graphics Processor: llvmpipe
Manufacturer: Gigabyte Technology Co., Ltd.
Product Name: H55M-S2H

If you open a nedit window, hit alt+F3, are there any signs of opening nedit on a particular virtual workspace under special window settings?

I don't remember seeing any such settings. Unfortunately, when I installed another Debian on another disk drive, the installer managed to make my original system unbootable, after promising not to affect it. Silly me. I should have unplugged the disk drive that I really wanted to protect. All the files -- especially my home directory -- are still there, but booting drops me into a single-user emergency session. So I can't check whether this was the problem. The new system sort-of works -- nedit works correctly, and evolution gets its new messages, but kalarm is messed up. So we can close this issue.