https://www.imda.gov.sg/regulations...-to-imda-standards/4g-services/jul---sep-2023
Based on IMDA testing, Singtel and M1 both failed coverage in the DTL for the last quarter reported. Won't be surprised if their upgrading to 5G and replacing 4G towers might have contributed to this.
Coverage will always change whenever they deploy new technology, because they will take down the old to replace with the new. This happened from 2G to 3G, and from 3G to 4G. And when they progressively upgraded 4G hardware over the years too.