Indeed, self-built works, buildbot file does not.
I don't see any notable difference in the files though.
(self-built)
binwalk kernel
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
0 0x0 uImage header, header size: 64 bytes, header CRC: 0xB486395F, created: 2020-04-12 19:35:00, image size: 2282708 bytes, Data Address: 0x80001000, Entry Point: 0x80001000, data CRC: 0xCB2D61FA, OS: Linux, CPU: MIPS, image type: OS Kernel Image, compression type: lzma, image name: "MIPS OpenWrt Linux-5.4.31"
64 0x40 LZMA compressed data, properties: 0x6D, dictionary size: 2097152 bytes, uncompressed size: 7163839 bytes
(from server)
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
0 0x0 uImage header, header size: 64 bytes, header CRC: 0x5B29C666, created: 2020-04-12 14:04:48, image size: 2433965 bytes, Data Address: 0x80001000, Entry Point: 0x80001000, data CRC: 0xA1879140, OS: Linux, CPU: MIPS, image type: OS Kernel Image, compression type: lzma, image name: "MIPS OpenWrt Linux-5.4.31"
64 0x40 LZMA compressed data, properties: 0x6D, dictionary size: 2097152 bytes, uncompressed size: 7684223 bytes