Why do spreadsheets look bad when I Ctrl+V them into an image editor?

I run a blog with both PDF files and images of puzzles. They used to look nice, but ever since I upgraded LibreOffice, they look ugly, and I have no clue what version of LibreOffice I was using before. I’ll try downgrading to 5.4.5 and see if that makes things better.

Here’s a section of the grid image for my Wordy Wednesday 208 as exported via Ctrl+V in LibreOffice

image description
The corners look really ugly.

The exact same image, as exported via Ctrl+V in my old LibreOffice:

image description
The corners are more rounded and nice.

For your experimentation, the ODT file: Dropbox - WW208-anacrossword15.odt - Simplify your life


Can’t seem to duplicate your posted view. Using the provided sample (Writer not Calc document) copied the image & loaded into Gimp (Gimp view is at 800%):

image description

Saved as .png and got:

image description

Then using Calc, just created some cells with borders, did Ctrl+C, Ctrl+V in Gimp producing (again at 800%):
image description

This was done on Mint 18.3 using LO v6.0.2.1