Are you sure they reverted back to Victorian times? Granted, some of the later movies are less obviously concerned with the War and are more generic Holmes mysteries, but I don't believe they ever
literally switched back to the gaslight era. (A foggy English estate, or dockside warehouse, looks much the same regardless if it's 1889 or 1945.)
For what it's worth, Wikipedia states explicitly that "the writers of the Universal series never reverted to the Victorian setting." Even if Holmes stopped fighting Nazis.
So no return to the "prime timeline" in that case either!
The last two movies produced did not mention a war, nazies, or anything past the 1900 mark. The first opens with Watson mentioning Irene Adler, implying the adventure with her occured between the last specifically war themed movie and that one.
They were based off of the books again, not new war themed stories, so even if Universal doesn't state it explicitly, it's the intention of the penultimate and final movies.
But Irene Adler never appeared in the Rathbone movies, so we have no idea when that timeline's version of her story took place. If Moriarity and Lestrade and Mrs. Hudson are all running around in the forties, there's no reason Irene Adler couldn't have been as well. Maybe Rathbone's Holmes stopped Irene Adler from blackmailing Winston Churchill.
And just because they didn't mention Nazis, and referenced characters from the books, doesn't mean that, hey, they're back in the 1880s again! Granted, it's been a few years since I've seen
Terror by Night or
Dressed to Kill, but are they really catching horse-drawn cabs and eschewing electricity in those films? And why would Universal, which had NEVER set their Holmes films in anything but the present-day (as opposed to the first two films from Fox) change course at that late date? My understanding is that they kept the movies set in the present but gradually segued away from wartime themes.
Universal often took a laissez-faire approach to settings back in that era. Just try to pin down exactly when their Frankenstein or Wolf Man movies took place; they're mostly set in some foggy, timeless, mythical Eastern Europe that has modern-day phones and trains--
and superstitious peasants and roving bands of gypsies.
And don't get me started on the chronology of the Mummy series, in which decades pass between the movies ("Twenty years ago, your father uncovered an Egyptian tomb . . . ") and yet it always seems to be the present-day 1940s!
Clearly, there was no internet back then. Can you imagine modern-day fans trying to make sense of those "timelines"?
("But wait! How can Moriarity be working with the Nazis in the forties when we clearly saw him stealing the crown jewels back in Victorian times? Canon violation!")