I am clueless why/how it happened.
The #genre column is the standard Calibre column that was hidden, I just unhid it
I changed #genre in all files to "Adult Fiction" then selected all my Dr.Who books to change them to SciFi.
I had originally ctrl+clicked 11 Doctor Who series's (880 eBooks) and it happened.
Just guessing but it looked to me (wishful thinking?) like 11 repeats by eleven repeats of the repeats?
I eventually unclicked 10 (so 1 selected) and entered "Adult Fiction" in the search field and NOTHING in the replace filed. The resultant field then became the Series Name (automatically)

and I fired it off.
Then I re-editted and entered the series name in the search field and "SciFi" in the replace field and it worked with that series only.
Had to go out so didn't take it any further. Maybe it didn't like the fact that I selected on Series then tried to search and replace the #genre field????
Will try again with 1 then 2 then more series when I have time to scratch myself