The prompt you get about the weapon needing maintenance is not a bug I think. You will only get that when trying to maintain a weapon which doesn't need it. (or when it actually runs out mid-battle) I guess the reason is that weapon maintenance removes the "sharp" status from a weapon, so it can refresh the duration properly.
Setting up a trigger to send some lited message to party status will tip you off when your weapon needs maintaining.
However that line did reveal a different bug, I'm not sure if it's with throwing weapons or weapon containers such as bandoliers and weapon sheaths, I've only tested so far on throwing daggers and bandoliers. But adding a maintained throwing dagger to a bandolier removes the "sharp" status from it.
You can test this by putting a maintained weapon into a weapon sheath, taking the weapon out and trying to maintain it again, you should not get the prompt about the weapon maintenance if you do.
I'm pretty bad at explaining things so I'll give an example of when I tried to maintain my sword twice in a row, and then my throwing dagger twice, after it had gone into a bandolier.
You carefully sharpen the bladed edges of your frozen sliver of iceron <deep white glow> until it looks shiny and new.
Your frozen sliver looks like it could use some maintenance. (prompt)
party report frozen sliver needs maintenance
You carefully sharpen the bladed edges of your frozen sliver of iceron <deep white glow> until it looks shiny and new.
You carefully sharpen the bladed edges of your meteoric dagger called 'shooting star' <deep red glow> until it looks shiny and new.
You put the meteoric dagger called 'shooting star' <deep red glow> into a dagger belt of the unseen assassin.
You get the meteoric dagger called 'shooting star' <deep red glow> from a dagger belt of the unseen assassin.
(wtf no prompt)
You carefully sharpen the bladed edges of your meteoric dagger called 'shooting star' <deep red glow> until it looks shiny and new.