It is a strange problem.
I noticed that some field values were mysteriously resetted to the default value.
Using Devel I tracked down the problem : the field values of some nodes are erased and replaced with default value when new nodes of the same content type are created.
In this case, the nodes are created programmatically through AJAX with a custom module. It often happens that the jquery event fires multiple times on a single click, which causes 2 to 5 nodes to be created almost instantly. Could it be that the extra nodes created would replace existing ones?
Thanks for any ideas