I boosted a friend's alt because they wanted to try out a different class. So naturally, I bumped my game down to Penitent to get him in. We ran some pits got him to 70. I had the idea of running a few Vile Splendor sigils for some starter gold for him, but I couldn't find them anywhere in my bag or stash tabs.
Then I noticed that I randomly had a bunch of Dungeon Delve sigils favorited. I thought it was weird as I have been salvaging them. I suddenly realized: "Oh this is exactly where my Vile Splendor's were in my bags. Wait this Sigil was my Treasure Breach I was saving for tonight."
I quickly ran to my stash to check on the Mythical Prankster I was saving, and boom it was suddenly a Dungeon Delve. I had 4 Vile Splendor, 1 Treasure Breach, and 1 Mythical Prankster. All of them gone: turned into Dungeon Delves.
I crafted 15 more sigils to get a Vile Splendor to confirm the hunch. I converted my game from T10 down to Penitent, and sure enough the Vile Splendor converted itself to Dungeon Delve. The Forgotten Wisdom sigil also converted down to a Dungeon Delve.
My Horadric Strongrooms and Obol Reserves did *not* convert down to Dungeon Delve.
I went over to Eternal to see if it's happening there. Crafted 10 on T4, swapped to Penitent, boom Vile Splendor to Dungeon Delve. I didn't get the screenshots of it happening on Seasonal, but I did on my Eternal character.
It was already reported on the bug tracker a few hours ago but another user. They claimed it was after a few days it happened to them. I do not know if that plays a factor but I know for a fact that difficulty swap from Torment >> Non-Torment makes it happen.
Tl;dr: Vile Splendor, Treasure Breach, and Mythical Prankster sigils are permanently converted to Dungeon Delve when swapping off of Torment Difficulty. There are probably more but those 3 are the main ones.