Strange change in behaviour when windows open for program already open

I run a TW KDE Plasma desktop and just recently something has changed as to where an additional window opens for an already running application.

To explain what used to happen.

a) I am running a browser in virtual desktop 1 and LibreOffice in virtual desktop 4, in the browser I download a comma separated value file from my online banking and am viewing the current file in LO on desktop 4.
b) A window would open on desktop 1 asking what I wanted to do with the CSV file, I would reply to open in LO
c) Next the confirmation of the CSV format window would open in desktop 4 and I would OK that
d) The new window would open in desktop 4 showing the contents of the new file in LO.

Now what happens is
d) The new window containing the new file opens on desktop 1 despite the other LO window is open on desktop 4

Now I hope that explains it well enough. I could understand if all windows opened in desktop 1 relating to the new file, although the obvious choice (for me at least) is for all the windows after b) above would open on the desktop where LO is already open.

Now I cannot see what I might have done or not done to change this. I know I could force LO to run on desktop 4 only but I don’t want that. What I want is for all windows to open on which ever desktop the initial window opens in.

Anyone any ideas please?