How do I get rid of this issue?

[warped otter]

this is a beveled curve of a very high poly model insctanced to another heavy scene.

• I couldn't bring the whole mesh data because every time i dragged it from my asset browser and ticked instace off, the model disappears thats why I couldnt identify any shading problems because there wasnt an accesible shader data for an instanced object from another project file. the model looks perfectly fine in its standalone file.

been scratching my head for hours for this, please help!!!

here some more pictures

[maiden jasper]

This might sound stupid, but have you tried both shade smooth and shade auto smooth? Often, one works but not the other

[warped otter]

I knew about this fix, didn't work. had to build the whole scene in the model parent file, a day of work turned into a week but it worked out i guess. here's the final result.