Stack svg layers with python

Question

I'm trying to stack different svg images as layers onto a base svg which is empty using python 3.9 and svgutils and svgwrite methods. The code gets all the svgs from one directory and images filenames are formatted as "n_x.svg" where n and x are variables; I should stack them following a path defined into this list:

unique_comb = random.sample(result, 100)
print(list(unique_comb[0]))

>>>['1_2', '2_3', '3_1', '4_1', '5_1', '6_2', '7_6']

This is a sample from a 100 unique combinations list. Here the iteration that stacks the svgs:

import svgwrite
import svgutils.transform as st

if __name__ == "__main__":

# PATH to svgs directory
os.chdir(PATH)

for i in range(0, 100):
    
    template = st.SVGFigure("400px", "400px")

    for j in range(0, len(unique_comb[i])):

        rand = unique_comb[i][j]
        layer_x = st.fromfile(f"{rand}.svg")
        template.append(layer_x)

    template.save(f"Image{i+1}.svg")

The problem is the following: append method is not working properly as it stacks 1 or 2 images correctly then it stacks random svgs from the source directory. I tried to debug it in some ways:

1. I tried to limit the iteration adding one image at a time and it gave as a output a strange combination of all the 7 svgs layers after stacking just 3 images (????).
2. I tried to reinitialize the base svg in every cycle of the inner loop.
3. I've stacked images without iterating and defining "manually" each layer (layer_1 =st.fromfile(f"{rand[0]}.svg"); layer_2 = st.fromfile(f"{rand[1]}.svg"); .....) and then appending them one by one.

It seems like append gets confused about which image it should stack and creates a messy result.

Which other way should I try to debug this method? Is there any other way to stack svgs iteratively?

Thank you so much