Why are most TCC & One-Line printing Issues solved by changing the Resolution (dpi) setting to lower than 600dpi?

Answer: It has to do with scaling, dpi and zoom factors. The lower the DPI, the better chance for the one-line or TCC to get the whole picture in.

Common High-DPI Issues
Applications that do not check the system DPI setting and do not adjust for the larger font and UI sizes can raise various classes of issues. This section describes the most common categories of issues and shows examples that illustrate them. The categories of high-DPI issues include:
Clipped UI elements or text
Incorrect font size
Incorrect layout
Blurred UI elements
Pixelated text and bitmaps
Misalignment of coordinate space affecting input
Partial rendering of a full-screen application
Incorrect use of effective resolution