My test case: echo -e 'a\u0308a'
Correct rendering: äa (That's what I see with: Courier, Cousine, DejaVu Sans Mono, FreeMono, Hack, Monospace)
Incorrect rendering: aä (That's what I see with: Andale Mono, Bitstream Vera Sans Mono, Courier 10 Pitch, Courier New, Liberation Mono, Mitra Mono, Nimbus Mono L, Noto Mono, Oxygen Mono, Tlwg Mono, Tlwg Typo, Ubuntu Mono)
My test case: echo -e 'a\u0308a'
Correct rendering: äa
(That's what I see with: Courier, Cousine, DejaVu Sans Mono, FreeMono, Hack, Monospace)
Incorrect rendering: aä
(That's what I see with: Andale Mono, Bitstream Vera Sans Mono, Courier 10 Pitch, Courier New, Liberation Mono, Mitra Mono, Nimbus Mono L, Noto Mono, Oxygen Mono, Tlwg Mono, Tlwg Typo, Ubuntu Mono)