Checkboxes values not visible in flattened PDF

[LKirst]

Describe the bug
convert_from_path() does not flatten tick boxes (and changes the appearance of radio buttons).

To Reproduce

from pdf2image import convert_from_path
images = convert_from_path("in.pdf")
im1 = images[0]
images.pop(0)
im1.save("out.pdf", "PDF", resolution=100.0, save_all=True, append_images=images)

Here is my test input pdf and the flattened output I generated:

in.pdf
out.pdf

Expected behavior
The checkbox that was ticked in my input file is no longer ticked in the flattened output. The round radio buttons have turned into boxes in the flattened output.