Grace notes in a chord

[Darius Blasband]

Hello,

I have a 2.2 score, recently migrated to 2.6.3, where I have something 
fairly common, such as:

\grace{a8[ b c d e f]} a4

which looked good, and which is printed as if the input was defined as a 
chord <a b c d e f a> and the resulting
music is closer to a mix between Frank Zappa and Stockhausen than my 
original intention.

The trouble is, I've been trying to reproduce the problem on a short 
snippet, and I have failed consistently.
The only thing I have is the complete document, and the resulting PDF.... 

Any idea anyone ?

Cheers,

Darius.