I'm sorry, I didn't find any explanations with the search feature, among the thread titles for the ser/estar feliz, so it was probably already answered, but it seems impossible to find. But, also, I would need more details.
"Ser feliz" is the recommended form, according to what I read in several forums.
For instance, this one: https://cvc.cervantes.es/foros/leer_asunto1.asp?vCodigo=37186
But, it's not clear if "estar feliz" is only a figure of speech, in some novels, or if it's as correct as "ser feliz".
Could it change depending on who is writing, and if they want to imply the permanency or not? Could it change depending on the dialect of the area? Or depending to the context?
Why "to be happy" is considered something inherent? Maybe it makes sense in Spanish, but in my own language, it doesn't, unless translating "happy" with something like "good tempered".
It's necessary to ask also here about the difference in the meaning and connotations with "contento", as it seems that "contento" is the same than "feliz" but in a more impermanent way? Like being happy or pleased of something?
Is that a possible explanation?