The small patch at https://salsa.debian.org/kernel-team/initramfs-tools/merge_requests/23 (https://salsa.debian.org/kernel-team/initramfs-tools/merge_requests/23) works fine.
Before: 100s of depmod: ERROR: ../libkmod/libkmod.c:515 lookup_builtin_file() could not open builtin file '/var/tmp/mkinitramfs_DkdaZa/lib/modules/5.4.0-2-amd64/modules.builtin.bin With patch: No Errors.