serial readUntil not working as expected

[martinwork]

With the code

    while (1) {
        ManagedString in = uBit.serial.readUntil(":");
        ManagedString out = "|" + in + "#\n";
        uBit.serial.send(out);
        uBit.sleep(100);
    }

sending ab:cd:ef:, V2 works as expected, returning

|ab#
|cd#
|ef#

but V1 returns

|ab:c#
|#
|ef#

Sample source and hex files
serial-readuntil.zip