I've tried both Fira Mono and Jetbrains Mono, it seems to barely get the two glyphs closers. Below are screen captures of Jetbrains Mono.
With ligatures: https://ibb.co/vQ3nCsd
without ligatures: https://ibb.co/DtD1P9X
But, to your point, it would seem it doesn't enable ligatures based on the parser, since there is a difference between ligatures active and disabled, and it applies on both lines.
I've tried both Fira Mono and Jetbrains Mono, it seems to barely get the two glyphs closers. Below are screen captures of Jetbrains Mono.
With ligatures: https://ibb.co/vQ3nCsd
without ligatures: https://ibb.co/DtD1P9X
But, to your point, it would seem it doesn't enable ligatures based on the parser, since there is a difference between ligatures active and disabled, and it applies on both lines.