Fallback font substitution ignored in Inkscape svg-export module

Inkscape's SVG export module invokes a fallback font mechanism when the originally referenced font is unavailable. Across versions 1.2.x and 1.3.x, a documented inconsistency exists: the fallback substitution does not reliably apply the font configured in Preferences → Fonts, leading to unpredictable rendering in exported SVG files. This behavior is further complicated by code-path divergence between GTK3 and GTK4 builds after version 1.3, where font handling logic differs. Default logs may omit the 'Font not found' warning, and increased verbosity is often required to capture substitution decisions. User reports indicate that clearing the font cache at ~/.config/inkscape/fontcache sometimes alters the outcome, pointing to an undocumented caching issue. The precise goal is to determine the conditions under which the user-specified fallback font is honored versus ignored during SVG export, and to identify the governing factors.

1. Under what circumstances does Inkscape disregard the Preferences → Fonts fallback setting during SVG export?
2. How does the GTK3 versus GTK4 build variant affect fallback font selection consistency?
3. What role does the font cache at ~/.config/inkscape/fontcache play in the substitution outcome?