I use Mac computers and a Canon Pro-100 printer. After recently updating my Mac desktop computer with Big Sur and Lightroom Classic to the 10.1.1 version, I can no longer make borderless 13"x19" (A3+) prints. In the past, I have printed dozens of 13"x19" borderless images.
The paper size options in the "Page Setup" feature of the Print Module no longer includes A3+ size. I have tried creating a 13"x19" print option under "Manage Custom Sizes," including reducing all Margins to 0.00 and turning on "Zoom to Fill." After making these adjustments, the screen shows an image without white borders, but the actual print is surrounded by ⅛" white borders.
I have tested the printer with my Macbook laptop, which is currently running under the older Catalina software and the older Lightroom Classic version 9.4. The "Page Setup" feature gives me the A3+ size and borderless options - and the prints are perfect and borderless.
The problem seems to be with the latest version of Lightroom.