model may be merged on multiple svg export

Here is a code that render 2 figures and export them in separated svgs:

```py
import cadquery as cq
from cadquery.cq import Workplane as WP
from cadquery.cq import Vector as V

cube1 = WP('YZ').move(1, 0).box(2, 2, 2)
cube2 = WP('XY').box(6, 3, 3)

def fig1():
return cube2.union(WP('XY').transformed(rotate=V(45, 0, 0)).box(6, 3, 3))

def fig2():
return cube2.union(WP('XY').transformed(rotate=V(0, 45, 0)).box(6, 3, 3))

def export(fig, fig_name):
cq.exporters.export(cube1.add(fig), fig_name, opt = {'showHidden': False})

export(fig1(), 'fig1.svg')
export(fig2(), 'fig2.svg')
```

The `fig1.svg` file is rendered as expected:


But in `fig2.svg`, fig1 and fig2 are merge together, instead of jut fig2.


If I comment the fig1 export line, fig2 is exported as expected: