I suggest that it actually is an initramfs-tools bug. As pointed out elsewhere in this very discussion, the program in question must be using file descriptor #

I suggest that it actually is an initramfs-tools bug. As pointed out elsewhere in this very discussion, the program in question must be using file descriptor #2 as its output file descriptor for the files that get corrupted, because stderred only inserts its hardwired control sequences on writes to that file descriptor. That's a bad thing to be doing independent of stderred, because there is potentially other (library) code in the relevant program (whatever it is) that assumes that file descriptor #2 is usable for diagnostics. Even without stderred in the mix, it is a bug waiting to be triggered via another route.