Unexpected ftp ack ListDirectory suffix

I’ve received an unexpected FTP ACK for a ListDirectory request for root (“/”) folder:

  • The data size reported in the message payload is 6
  • The data itself should be a list of null-terminated entries in the format [D|F|S]<filename>\t<file size>\0...

Instead I’m getting the following data:
[68, 65, 80, 77, 80, 169]
The first 4 bytes make sense, as they translate to DAPM indicating it’s a [D]irectory called APM (which does exist). However, the next byte is unexpected - it’s not tab (\t = 0x09) but rather the value 80. The next byte is also unexpected: 169. The ListDirectory docs also don’t specify what would be the “file size” in case it’s a directory although MAVProxy code indicates there is not tab at all - just the name of the directory, however, that’s not the case either (no null term after the dir name here…)

Any idea what’s going on?