Hi gert,
We have checked the reported issue on our end. Upon further analysis, our HTML converter uses the Blink rendering engine, which in turn utilizes the Chromium executable in headless mode for converting HTML to PDF. This conversion process preserves the PDF document layout as it appears in Chromium-based web browsers (such as the Chrome print preview).
Upon testing the provided HTML document directly in Chrome's print preview, we observed the same behavior. Please refer to the attached screenshot for reference.
Screenshot:

We recommend using colors with lower opacity to create a thinner table border by adjusting the CSS styles.
Regards,
Arumugam M