Above-base vowel signs have narrow forms in some contexts. A tsa ’phru blocks an above-base vowel sign from being narrow when they are not in adjacent clusters. In the first example string, U+0F59 in the first cluster widens the third and fourth clusters’ vowel signs, despite the presence of the second cluster. The second example string shows how the vowel signs are narrowed without the meddling tsa ’phru.
Character data
ཙཀཀོ་ངོ
U+0F59 TIBETAN LETTER TSA
U+0F40 TIBETAN LETTER KA
U+0F40 TIBETAN LETTER KA
U+0F7C TIBETAN VOWEL SIGN O
U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG
U+0F44 TIBETAN LETTER NGA
U+0F7C TIBETAN VOWEL SIGN O
ཅཀཀོ་ངོ
U+0F45 TIBETAN LETTER CA
U+0F40 TIBETAN LETTER KA
U+0F40 TIBETAN LETTER KA
U+0F7C TIBETAN VOWEL SIGN O
U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG
U+0F44 TIBETAN LETTER NGA
U+0F7C TIBETAN VOWEL SIGN O
SKing-2003
commented
2 years ago 
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
Above-base vowel signs have narrow forms in some contexts. A tsa ’phru blocks an above-base vowel sign from being narrow when they are not in adjacent clusters. In the first example string, U+0F59 in the first cluster widens the third and fourth clusters’ vowel signs, despite the presence of the second cluster. The second example string shows how the vowel signs are narrowed without the meddling tsa ’phru.
Character data
ཙཀཀོ་ངོ U+0F59 TIBETAN LETTER TSA U+0F40 TIBETAN LETTER KA U+0F40 TIBETAN LETTER KA U+0F7C TIBETAN VOWEL SIGN O U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG U+0F44 TIBETAN LETTER NGA U+0F7C TIBETAN VOWEL SIGN O ཅཀཀོ་ངོ U+0F45 TIBETAN LETTER CA U+0F40 TIBETAN LETTER KA U+0F40 TIBETAN LETTER KA U+0F7C TIBETAN VOWEL SIGN O U+0F0B TIBETAN MARK INTERSYLLABIC TSHEG U+0F44 TIBETAN LETTER NGA U+0F7C TIBETAN VOWEL SIGN O
Screenshot