In a rather long session of ONI I got Sweaters for almost all my duplicants, but upon discovering another bug (valves) I saved the game and continued the last autosave and of the 8 duplicants that had a sweater by then, only 2 remained (10 dupes total). At least I managed to replicate it.
I can provide the saves, but since it might have something to do with the saves, I'm not sure if it would help, since the bug happens either when saving or when loading.
Like mentioned below, the number of clothes in the item counter to the right will update only after loading a game and only counts the physically available clothes, excluding the ones worn by duplicants. If the number of those "ownerless" clothes is lower than the number of clothes worn by duplicants, duplicants will loose their clothes upon loading of the game.
Steps to Reproduce
You need to produce some sweaters (lets say 4-6 of the Warm Sweaters). You want to equip half of them to your dupes. Notice that the number of sweaters under "Clothes" in the menu still includes the ones you already handed to your dupes (and even if there would be none lying at the Textile Factory, you would focus on that spot as if they were still there). Now you need to save the game as whatever you want to name it. Exit to menu, load the save. Your duplicants should be fine, still wearing sweaters, but the "Clothes" value updated, to represent the actual number of clothes left at the Factory. If you now go ahead and equip the rest, save, exit and load you will find out that, since the value updated upon loading to be 0, no Duplicant is wearing any clothes anymore.
This issue has not been confirmed by a developer yet.