The font for this test case has an AAT Extended Glyph Metamorphosis Table with a single Contextual Glyph Substitution Subtable. Its finite-state machine contains two states (0 and 1). In state 0, glyph A moves to state 1; glyphs B, C, D, E get replaced by an encircled form. In state 1, no substitution happens. If your implementation is correct, the input string AB should not get any substitutions, but a stand-alone B should get rendered inside a circle.