The patch for T5834 (https://dev.gnupg.org/rMad3aabdd8a64156c7e3a75d695ae1ab2c4bec841) was already applied to the build of GPGME 1.17.0 for Focal, as I did browse the list of latest GPGME bugs first before reporting this bug. Attempting to build with the latest GPGME 1.17.1 (with the included ABI patch) results in exactly the same FTBFS for i386 only, so this does appear to be a distinct issue not related to that of ABI backwards compatibility.