Mysterio reportedly signed a two-year deal with WWE last month. He had been trying to negotiate an 18-month contract but WWE wanted a two year deal, which they settled on. Mysterio did manage to work in a clause where you could give notices at the 18-month mark if he wants to leave.
Mysterio returned to WWE at the Royal Rumble earlier this year and later worked the Greatest Royal Rumble event in Saudi Arabia in April. The two sides have been working on a deal since then.