Isabeau is gorgeous.
Love it.
I'm starting to tire of
Isabelle and Isabeau is much more refreshing.
But Isabeau was never masculine, despite the "-beau" in there. In medieval
France, one way to make a feminine nn was to have it end in an "o" sound:
Marguerite -->
Margot,
Mathilde -->
Mahaut,
Marie -->
Manon,
Isabelle --> Isabeau. I have no idea why the spelling of the "o" sound varies so much.