Please share the output of "pdfly pagemeta" https://pypi.org/project/pdfly/ of the first page of that document

If you look at the pdf, you will see the polyline. To make it visible change the color. The problem is just the default color. Set it to red ie. ArrayObject(FloatObject(1.0),FloatObject(0.0),FloatObject(0.0)) and it should be good

Interesting, I would have thought that black is the default color.

Here is a complete example:

from pypdf import PdfWriter
from pypdf.generic import AnnotationBuilder, NameObject, FloatObject, ArrayObject

writer = PdfWriter()
writer.add_blank_page(200, 200)

annotation = AnnotationBuilder.polyline(
    vertices=[
        (50, 50),
        (50, 150),
        (150, 150),
        (150, 50),
        (50, 50),
    ],
)

# Change the border color of the PolyLine:
annotation[NameObject("/C")] = ArrayObject(
    [FloatObject(0.9), FloatObject(0.1), FloatObject(0)]
)
writer.add_annotation(page_number=0, annotation=annotation)

with open("output.pdf", "wb") as f:
    writer.write(f)

From the specs:

It seems that the problem is on the side of the visualization of the pdf on Mac. I don't see the annotation using either Preview or Chrome, but I do see it using an online pdf reader.

Is the new code I shared also causing those issues or is it just the old code?

The code you shared did not solve the issue.
If that helps, I also tried with the rectangle annotation and had a better result:

from pypdf import PdfWriter
from pypdf.generic import AnnotationBuilder

writer = PdfWriter()
writer.add_blank_page(200, 200)

# Add the rectangle
annotation = AnnotationBuilder.rectangle(
    rect=(50, 150, 150, 50),
)
writer.add_annotation(page_number=0, annotation=annotation)


with open("output.pdf", "wb") as f:
    writer.write(f)

The generated pdf has a visible annotation on Chrome but not on Preview (which seems to be known for poor annotation support).

if the issue in on Preview, I think we can close this issue

Let's leave it open to either document the color-thing or change the default value (which is blocked by the annotation PR #1745). This will be pretty easy to close once the annotation PR is merged (I now hope for mid of May)