According to RFC 4648, the base64 encoding "implementations MUST NOT add line feeds to base-encoded data". See https://tools.ietf.org/html/rfc4648#page-3 However, there is a line feed being inserted by busybox base64 encoder every 76 characters. This is incompatible with other base64 implementations which are RFC compliant, such as the GNU implementation.
In my testing, base64 matches GNU coreutils behavior: $ base64 --version base64 (GNU coreutils) 8.30 ... $ base64 <busybox_unstripped.map >1 $ ./busybox base64 <busybox_unstripped.map >2 $ ls -l 1 2 -rw-r--r-- 1 root root 1927550 Apr 20 14:40 1 -rw-r--r-- 1 root root 1927550 Apr 20 14:40 2 $ md5sum 1 2 b38f09d47ed8b4edcf099104481bca7d 1 b38f09d47ed8b4edcf099104481bca7d 2