Thanks for the answer. Is a workaround existing ?
Choosing sorting by manual (vs *gallery image sort order) in dynamic folders totally change the order, how does it work ?
Then the order is constant, but unpredictable.
Choosing the dynamic folder thumbnail work very well.
Another question, is it possible to change the search parameters (tags) used by a dyncamic folder, after it was created ?
The only way I have found is to delete it, and recreate it.