DD fails beyond 2GB seek
Don't use busybox dd (compiled with bionic libc). Instead you may use static dd binary from gnu coreutils compiled with glibc or musl. On my device all binaries work well; Android's toolbox dd, meefik's busybox dd and statically built dd.

Bionic is confirmed to be buggy with large files.

PS: Playing with eMMC is dangerous, particularly when testing buggy dd. dd is a killer, in my opinion. You can accidentally overwrite initial few bytes of mmcblk0 rendering your device useless.