Case by case basis I think. There is a section for this but I can't recall section. Likely Penal Code offence. End of the day, AGC has prosecutorial discretion on this.
For instance, you are aware Ah Loong operates a gambling den somewhere near River Valley. Even chut '48' against the gamblers while operating his den. You choose not to report. I am sure AGC won't prosecute you for not reporting a seizable offence.
On the other hand, if you witness Ah Lian kena raped by your friends after a party at her condo somewhere in Upp Bt Timah and act blur, once Ah Lian lodges a police report, you will likely be charged for not reporting the offence of rape.
If Ah Loong kena raped, and you don't report, they may even sentence you to death.