No, it wasn’t the Victoria or the Lauridsun; I think it starts off in g minor chords... and often went from 4 to 6 to as many as 8 parts...I can hear the thing in my memory as a wonderful sonorous blend. The other thing that sticks out is there is a there is a slight pause from the magnum then the Et amirabile part comes in in with the tenors and altos splitting with the tenors on e and the altos on a” and their part is in a minor with the tenors singing 5ths from the alto’s a(key signature changes in that section to c I think in that section)….the rest is lost some what to memory but I just know if I ever heard it again I could sing my bass parts perfectly....the song is like the “lost chord” for me. It was the first big adult serious song from highschool I had ever sung, and I had thought I had touched upon Heaven’s harps and felt humbled to be privileged to sing such music. My choir master was David Buttolph who for years was the orchestra and choir leaders at Binghamton university(then Harpur College at SUNY). He said I got in because he’d never seen an ear like mine; he’d play 5,6 8 note cords and I could pick them apart vocally; no I couldn’t always tell what sort of major or minor 6th to the nth power removed but I knew generally what major or minor chord they were by hearing them,(”aflat-a- c and e” singing them out.
Anyway my impression is that it was more of a Rutter sounding piece or someone of his time and philosophy....more modern 20th century but not bizarre in rhythm or atonic in sound. Just a lovely sonorous work. I have an ache in my soul to hear it again but I can not for the life of me remember the creator of the work.
I do not recognize the description, especially since you are describing a divisi (eight part) piece. The only prominent one I found that seems to match at a glance is the first one; I have included links to a recording and sheetmusic. After that I have listed some lesser possibilities. Following further links I included some of the polyphonic era composers. (Maybe there will be something you like, if you cannot find the one you wanted.)
Frank La Rocca
https://www.giamusic.com/store/resource/o-magnum-mysterium-satb-divisi-print-ww1406
Francis Poulenc
Pierre Villette
Ola Gjeilo
Pascal Picard
www.musicanet.org
https://www.sheetmusicplus.com/
http://www2.cpdl.org/wiki/index.php/O_magnum_mysterium
Johann Stadlmayr
William Byrd
Nicholas White
Pedro de Cristo
Giovanni Pierluigi da Palestrina
Clemens non Papa
Giovanni Gabrieli
If nothing suits you, I suggest you do a search for: “O magnum mysterium” SATB divisi.
You will come up with numerous more obscure settings, but maybe one of those is it.