Khmer Unicode Font Error: unable to type two vowels for one consonant

I can't type two vowels for one consonant in a newly created document of both MS Word and OpenOffice for Mac OS Sierra. I couldn't type the word below pronounced as /pei/.

I tried both ways below but it didn't work.

Surprisingly, it worked for the word 'eat' in Khmer.

This problem didn't happen when I typed the word in Chrome's address bar. I tried to type the word using the fonts Khmer OS System, Khmer OS Content, Khmer OS Battambang, Koh Santepheap, and DaunPenh in MS Word and OpenOffice but still no luck. At last, I found one that worked for me. It was Kh Battambang. I think it came with Mac OS by default because it showed in the Computer list in Font Book application.