Why? Because . A keyboard layout tells the computer which code point to send. The Pyi Htaung Su font expects specific legacy code points (often Burmese script placed in the Latin-1 or ASCII range). If you type "a" on your keyboard, a modern Unicode font might show "ခ," but Pyi Htaung Su might show a completely different Myanmar character.
Type ဘ (The 'ဘ' will automatically jump under the 'မ'). Type ာ Common Keyboard Shortcuts (Standard Layout) pyi htaung su font typing