The Earmuffs accessory originally had a unique two-part unlock condition. Part 1 was to complete the Bounty Hungers event while it was active (which I did), triggering an unlock notification with just a thumbnail of the earmuffs. Part 2 was to own the Halena & Cook Character Pack, which would have triggered a second unlock notification showing Halena and Cook's sprites with the earmuffs equipped. However, I intentionally disabled the DLC since release, so that my main file would persist in this halfway state for future testing.

The version 3.13.3 update on August 28th removed the DLC check for unlocking the earmuffs, so I made some extra backups and decided to wait until Halena and/or Cook became available in the free character rotation. (I tested this scenario several times in the past, and was not able to equip the earmuffs at the time.) What's interesting about the 3.13.3 update is that I never got the second unlock notification on my main file, and yet the earmuffs are unlocked.