When in third person, if you have a modded shield or curio item on you, it will make celestisynth weapons look see-through. Examples:

it seems to work fine if you change the texture and model files to act like vanilla weapons. Example of breezebreaker retexture:

Works fine if no items are equipped or in the offhand.

The issue must be with the weapon models, maybe to do with the fact they are flat 0 width textures. If the width is increased this may be fixed.

the same