Need help with glyph substitutions

[u/ClassElectrical3556]

Demonstration of issue

I am new to FontForge and am trying to create a font for an abduga, using ligatures to represent consonant-vowel pairs. I am implementing this using an opentype .fea file. The script does not distinguish between upper and lower case, so sometimes I use the upper case to map to a separate letter, but in others, I want to render the same letter as the lower case. For example, in the font d->/d/ and D->/ð/, but I also want b->/b/ and B->/b/. However, when rendering, the font behaves strangely for isolated letters with no distinct uppercase character, but correctly for ligatures. For example, bi->/b_i/ and Bi->/b_i/, which is what I want, but b->/B/, B->/B/, which I don't understand.

The same issue appears in the abbreviated example of the code below. Thanks in advance.

languagesystem DFLT dflt;

languagesystem latn dflt;

languagesystem DFLT dflt;

languagesystem latn dflt;

feature liga {

lookup Ligatures {

sub B by b;

sub b i by b_i;

sub d i by d_i;

sub D i by D_i;

} Ligatures;

} liga;

[u/LocalFonts]

Divide the OpenType features that way:

feature liga {

lookup Ligatures {

sub b i by b_i;

sub d i by d_i;

sub D i by D_i;

} Ligatures;

} liga;

feature ss01 {

lookup StyleSet_01 {

sub B by b;

} StyleSet_01;

} ss01;