I just got a Partisan of Light, used it in battle a few times and then repaired it. After repairs it now longer shows up in the drop down as a possible weapon to fight with. I have the stats for it and all my other weapons that I can use are showing up just fine.
Do you have any MP? Even if you're not a magic user, you can still use some weapons that require MP to use. I know I had this weapon randomly appear and disappear and it was because of MP
Squid's probably right. Some "normal" weapons will use a bit of MP every time you use them in battle, and when you're MP runs out, you no longer "meet the requirements" to use it, so it doesn't show in the list. If you recharge your mana at the temple, it should solve the problem.
Hopefully it's just that and nothing more serious
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