1/4 notes stems don't merge

[Jean-marc LEGRAND]

Hi list.

Using Lily 2.8 on XP.

When I run this :

{a8 [a16 s a16 a8] }

I get the a16 stems "unlinked".

How can I get the same thing as {a8 [a16  a16 a8] } ?

I get the same pb using \hiddenNotes

Best regards

JMarc