\concat + \line = unexpected behaviour [?]

[Kieren MacMillan]

Hello all,

In the third line of this snippet, all whitespace (word spacing) is removed -- 
is this expected behaviour?

\version "2.13.21"
\markup { \line { in a line } }
\markup { \concat { pre "in a line" post } }
\markup { \concat { pre \line { in a line } post } }

Thanks,
Kieren.