Using DejaVu 2.34 (and I can't see any changes since then), the Sans Mono subscript i (ᵢ) looks like a subscript 1 (₁). It's possible to tell them apart by direct comparison, but only just. It seems to me that what is needed is to ensure the dot does not merge with the stem at small sizes.
If there's some way I can help fix this (I have a basic familiarity with type design and know how to use FontForge!) then I'd be happy to attempt a patch.