Yes, Duterte didn't like us, whether or not it was justified (I had heard it was an actual operative that was removed from custody while he was mayor or governor, but your story seems more reasonable).

Plus, Trump basically dumped the Asia/Pacific region. China was the only one that could step into the void, and they did so with relish. I believe it was during the Trump years that China drew their dotted line (similar to Libya back in the 80s...their "line of death"), and the US did little to oppose it because Trump didn't care.

However, it was about the time that they lost the ruling in international court that they started building their artificial islands (2013). Obama (then POTUS) did nothing. So now they have several outposts in international waters (and I think one in Philippine waters) that they have to right to, and dislodging them would involve blood. Not a great option.