When an user type a Unicode text in a sign, the length is limited using byte count, not real character count.
That means that if each Unicode character uses 3 bytes (like Korean or Japanese) it will show only 5 characters (15 / 3).
This has been tested in both vanilla MCPE multiplayer servers and PocketMine-MP servers. The packet sent (SignUpdatePacket 0xb6) is correct and contains the correct text. The packet dump is shown here: https://gist.github.com/shoghicp/ecf83b6e92a8a9147805
So, the client only gets the first 15 bytes, instead of the first 15 characters.
Local worlds are not affected by this issue.