Oddly, I just noticed that
Isabella shows a similar pattern on the SSA list (off it for the same 40 year gap as
Isabelle, then back in 1990), but
Isabel has never been off the list, just hovering at #400-500 for those years, and a correspondingly more gradual recent rise.
I wonder why that is? Why would
Isabel be used when
Isabella and
Isabelle were clearly not? Maybe that is the subject of another thread?