When the Summer Festival ended, and the Summer Artisan was removed or hidden, it also removed or hid the final Armorer in my list, Elsbeth Missaglias.
However, "Manage Artisans" still counts (correctly) that I have three Armorers, while it only displays two: Bitrid Brasshammer and Tomaso Missaglias. And it counts that I have 25 total, while it only displays 24. Unfortunately, Elsbeth is not available for crafting.
I suspect she is still there, but hidden along with the Summer Artisan.