I know I didn’t write this post but not necessarily is my answer as my friend who is an [name_f]Isobel[/name_f] goes by [name_f]Bella[/name_f] and my other friend goes by [name_u]Izzy[/name_u] (there is multiple of them where I live) and they both chose the nns themselves.
I don’t think she would be called that. If you do like [name_f]Belle[/name_f] as a nickname, maybe you should use the spelling [name_f]Isabelle[/name_f]. I’ve known multiple Isabelles/Isabels and Isabellas and the majority of them went by their full name.