I'm just wondering if you told the immigration guy something different than before.
There are many people coming to Singapore and back everyday for work but I guess that's different.
I knew there was a lady who came all the way from KL to join our sports volunteer work.
I met her in April when where are doing the HSBC Rugby 7s.
You could check https://www.volunteer.sg/ and see if there is any opportunities when you come.
If not, maybe you can go Bintan or Batam in Indonesia