I remember discovering that while writing some code for a job interview. The reason for it is simple, even though in many input systems (like the ABC International I use on my Macs) it’s a two-character sequence to enter ł, there is not actually a combining character for that line through the l. I’m not sure, but I think sqlite’s remove_diacritics works the way that I’ve implemented that functionality in some of my own software: convert to NCD then remove combining characters from the string. I would expect that a few other special cases also behave the same way, such as ħ or ø which also will not decompose.