After some tests I found that the reason of the behaviour described was: not explicity setting the column prefWidth.
If you create a Tableview with data inside since the beginning and without setting column prefWidth, all columns width will match the data maximum width and, if you change column width, as soon as you drag one the width will be restored to the original value.
If you create a Tableview with data inside since the beginning and setting column prefWidth, all columns width will match the setted value and if you enlarge a column before dragging one then that column width will remain unchanged.
I have no idea if this is a bug or it is wanted. However thank you jewelsea for the suggestions.