The lookup narrow_tsheg only applies to U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG, not to U+0F0C TIBETAN MARK DELIMITER TSHEG BSTAR. However, the font should treat them identically. The only difference between them is their effect on line-breaking, which is not relevant to a font. The simplest and best way to fix this is for 'cmap' to map both code points to the same glyph.
Character data
ཐོ༌ངོ
U+0F50 TIBETAN LETTER THA
U+0F7C TIBETAN VOWEL SIGN O
U+0F0C TIBETAN MARK DELIMITER TSHEG BSTAR
U+0F44 TIBETAN LETTER NGA
U+0F7C TIBETAN VOWEL SIGN O
ཐོ་ངོ
U+0F50 TIBETAN LETTER THA
U+0F7C TIBETAN VOWEL SIGN O
U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG
U+0F44 TIBETAN LETTER NGA
U+0F7C TIBETAN VOWEL SIGN O
Font
NotoSerifTibetan-Regular.otf
Where the font came from, and when
Site: https://github.com/googlefonts/noto-fonts/blob/b7dab18090b095dc82f1db789abd81a8e4b322ad/unhinted/otf/NotoSerifTibetan/NotoSerifTibetan-Regular.otf Date: 2022-03-01
Font version
Version 2.101
Issue
The lookup
narrow_tsheg
only applies to U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG, not to U+0F0C TIBETAN MARK DELIMITER TSHEG BSTAR. However, the font should treat them identically. The only difference between them is their effect on line-breaking, which is not relevant to a font. The simplest and best way to fix this is for 'cmap' to map both code points to the same glyph.Character data
ཐོ༌ངོ U+0F50 TIBETAN LETTER THA U+0F7C TIBETAN VOWEL SIGN O U+0F0C TIBETAN MARK DELIMITER TSHEG BSTAR U+0F44 TIBETAN LETTER NGA U+0F7C TIBETAN VOWEL SIGN O ཐོ་ངོ U+0F50 TIBETAN LETTER THA U+0F7C TIBETAN VOWEL SIGN O U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG U+0F44 TIBETAN LETTER NGA U+0F7C TIBETAN VOWEL SIGN O
Screenshot