In short: When running the xochitl tweaked binary through the RM2FB server, switching between notebooks and other recent documents, including PDFs, the page does not render the template background correctly, and PDFs will render with distorted color shading.
Please see the below-linked issue, as it describes the issue and behavior in full.
https://github.com/ddvk/remarkable-hacks/issues/518
In short: When running the xochitl tweaked binary through the RM2FB server, switching between notebooks and other recent documents, including PDFs, the page does not render the template background correctly, and PDFs will render with distorted color shading.