\mergeDifferentlyHeadedOn made strange behavior in chord, or it's a bug?

[cc0_knight]

Hi everyone!

as the code mentioned below, when i try to engraving, the root note of the chord are not same as upper note. how to solve this problem?

the aim:

problem:

code:

\relative c'' {

  \mergeDifferentlyHeadedOn

  \time 4/8

  <<

    {<c e>2}

    \\

    {<c e>8 [g'8]}

  >>

}

best regards!