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

[cc0_knight--- via LilyPond user discussion]

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!