At the same time, it is possible some error happened in your STP application. So, appeal mite help.
I did my PhD in Singapore. And my EP was rejected not once, but twice. Both due to misinformation in the application. Thanks to HR.
And when I applied for PR later, I got it in one attempt despite the bad experience with EP.
I hope there is a minor issue with ur STP and you get the opportunity to live and work in SG.