Conflicts with Google Input Tools. Solution: Disable other language input methods in Windows Settings (Time & Language > Language > Preferred Languages). Let Keyman be the sole manager for Sinhala.
While Nidahasa 3.0 is frequently associated with Windows, the underlying Keyman engine allows for Sinhala typing on macOS, Linux, Android, and iOS. How to Install and Use Tavultesoft Keyman Nidahasa Sinhala 3.0 By Uthmax