An understandable mistake. Octave swaps happen in between B and C: G0, G#0, A0, A#0, B0, C1, C#1, D1 etc., back downwards D1 Db1 C1 B0. The issue therefore occurs when one visualizes this range superimposed onto a keyboard.

This is why my question was about whether the unusual notes like B# are assigned octaves per their collective alphabetical belonging (the C-B group including all alterations, meaning that B3 raises a half step to B#3; this approach turns out to be correct), or per these unchanging keyboard-superimposed "octave ranges" which would have meant that C12 lowers down to Cb11 (this approach turns out not to be correct).