I added some common abbreviations as aliases to help with entering grammatical features, some work, some don't. For example, "sg" (singular) and "nt" (neuter) don't work, while "pl" (plural) does.

"singu" is now working for some reason, but "sg" and "nt" still don't

This seems to be part of a general problem of language fallback not working when searching for grammatical features. When using Bengali as the interface language, for example, searching for "singu" does not return "singular" as one might expect; one has to search for "একবচন" to get the same item.

I suspect that Nikki using British English as an interface language and not being able to use "sg" (an alias in "en", "de", and "cs" but not "en-gb") is another manifestation of that.

I don't think language fallback explains the behaviour I'm seeing here since I was using English at the time I originally reported this (I switched to British English more recently). But now that you mentioned it, I just tried switching back to English and searching for "singu" is actually still failing when the UI language is set to English, even though it works for British English and the item has the same label for both. "sg" and "nt" don't work even when the language is set to English.