Rightfully, they have the right to intervene you because the normal 30-day limit is for social visit (as already stated in the stamp). An immigration officer once told me that out of the 30-day limit, Singaporeans are only allowed to reside for a maximum of 21 days per calendar month. But I don't know if this is his idea or is official? As for me I don't have this problem because we have both MACS and MM2H.Recently when I went back home to JB with my family the CIQ officer said that we have no permit to stay in JB and can charge us with illegal staying. Is that just trying to scare us or is that really the case? We did not overstay the 30 days allowed by the social visit stamp per entry so I can't see where the illegal stay part is. Anyway the MM2H is not a financially feasible option for me now. Have you had the same experience before? Should remain calm or panic?
You should have planned to apply for the MM2H when you first step in to buy your house in JB because it will then give you a peace of mind in case they flip their policies, especially when Mahathir is now the new PM. You will know that he always has an axe to grind with Singapore when he was the PM previously. So, history may repeats itself again.