There are multiple problems with decks.
First, it seems that you now trim deck name.
I do prefer “Theory ::Mathematics ::Algebra ::Group ”, but names were changed to “Theory::Mathematics::Algebra::Group”
This is a problem mostly because, without space, it makes very long non breakable lines, which breaks the HTML rendering of question (I show deck name in question)
I have not yet understood what exactly did occur, but I now suddenly have a lot of decks which are duplicated.
On ankidroid and older anki, I see toplevel decks “Theory” and "Theory ". “Theory” have some of the subdecks while "Theory " have some other subdecks. I had not yet time to learn rust, so I can’t read code to check my intuition, but I believe that what ocurred is probably that the name "Theory " was corrected to “Theory” but the name of “Theory ::Mathematics ::Algebra ::Group ” was corrected into “Theory ::Mathematics ::Algebra ::Group” and displayed as “Theory::Mathematics::Algebra::Group ”
As a related note, the default deck is back in the deck selection window, even when it is not shown in the deck browser
Unrelated note, please let me know if I should put it elsewhere: the window to reply is far too small. My reply is long, and a very small window is not really practical. It seems I can’t change it’s size.
By the way, I can’t sync anymore. It keeps telling me to check the database, and even if I check it without add-on, and try to sync again, it tells me to check the database again. I believe that both problem may be related, but may be entirely wrong.
The trimming happens on each component, so simply updating should not result in duplicates. It’s likely caused by having the old deck names on your other clients and them getting mixed together - reverting to a backup and then forcing a full sync immediately after upgrading will likely prevent that from happening. If that does not also resolve your syncing issue, please let me know your AnkiWeb ID in a private message.
Trailing spaces are usually user error rather than a deliberate choice, so rather than restore the old behaviour, I’d recommend you use a little Javascript on your card to modify the displayed deck name to your liking, such as replacing the :: with a space.