It's there in April as we have some flights booked on it (to ROR) and they are still showing active and it's still on timetable as of today for me in April and May and such. As far as CO.com goes I'm able to book it for the dates you say you are not able to book it (the may dates) and it shows availability, did you check all fare classes for availability or just one class?

However, a switch in schedule for a little bit doesn't surprise me out this way, there could be runway work being done or they could alter their time frame or whatever. There could be a cultural issue going on so the flights to ROR are full up or filling up and they move you over to a direct flight from MNL to GUM etc..