I'm not really sure. I did take a couple of slips of exposed albumen paper and tried one one way and one the other, and the one that I did selenium first then fixer lost more density in the toner than the one that I did fixer first, so I suspect that's the reason.

Generally, one exposes albumen paper so that the untoned print is about 1.5 to 2 stops "too dark," because it lightens up in the toner, then lightens more in the fixer, though it then has some dry down, so it gets a little of the density back.

I suppose another issue might be that gold-alkali toners are alkaline, and fixing first might more quickly exhaust or contaminate the gold toner, where that wouldn't necessarily be a problem with selenium (since we do it all the time with regular silver gelatin papers).