Currently, Myanmar text is rendered as boxes. Example: ပြည်ထောင်စု သမ္မတ မြန်မာနိုင်ငံတော်
The Unicode block range for Myanmar is 0x1000 to 0x109F. There are also other two extended block for Myanmar, 0xA9E0-0xA9FF (Myanmar Extended-B) and 0xAA60-0xU+AA7F (Myanmar Extended-A).
iOS doesn't ship with any fonts that support Myanmar. If one such font is installed via a configuration profile, the fix for https://bugs.webkit.org/show_bug.cgi?id=138906 should fix this.
This is no longer a WebKit bug.