I think I pieced together the reasons why it's normal :
- commandLink's f:param's are submitted to the server (I didn't think so) and are part of the requestParametersMap
- f:viewParam's do the entire JSF livecycle, even on postbacks
- f:viewParam's then update their values, with what's in the requestParametersMap
- if the requestParametersMap happens to have keys corresponding to viewParam's names, the viewParam does its thing (uses the value to update the model)
So... that's normal. I'll modify this behaviour with stateless viewParam's (like OmniFaces' o:viewParam) : no update on postbacks, no more problems !
But I don't know why my problem appeared with mojarra 2.1.27 whereas everything seemed fine with 2.1.25. Well...