I've noticed this before but finally made an effort to save and confirm and test. Unfortunately this party was literally built around having Elemental Weapon on the Fighter!
Images are of Elemental Weapon. If you open the images you might see the stats for an item you've never seen before.
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https://imgur.com/a/rgnlx2j[/img]
--Spell cast in Camp.
--Game in turn-based mode.
--No dipping, no other weapon effects applied. Caster is not doing anything, wielder is not doing anything--nobody does anything, I just hit Leave Camp.
My game is heavily modded but as far as I know those mods wouldn't have touched Elemental Weapon or Magic Weapon in this respect.
I also tried casting Elemental Weapon on the melee weapon and Magic Weapon on the ranged weapon--both spells disappeared.
I have tried this from different save points.
I sort of know that there are issues with zone changes and maybe this is one of them...?
Does zoning break concentration?