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.
Some users want to be able to add cards to the default deck: Backend.get_deck_names doesn't return Default deck
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