As I've heard, they're signed through the end of May/beginning of June.

I will put it in caps because that's how important this statement is:

IT DOES NOT MEAN THEY'RE LEAVING AT THE END OF MAY.
IT DOES NOT MEAN THE SHOW IS CLOSING AT THE END OF MAY.

May is just when their contracts expire, as they've signed year-long contracts from when they began rehearsals. They, like every other actor, have the option to extend, if the producers offer them an extension. If they're not offered an extension than, YES, they're leaving when the contracts expire.