You have asked Modeller to read only one chain of the dimer (the A
chain), so it has quite happily gone and done that, then aligned your
sequence to it. ;) Change LAST:A to LAST:B (or remove model_segment
entirely, which will have Modeller read the entire PDB file).
Note that Modeller's alignment functions ignore chain breaks. So your
final alignment might need some manual adjustment to get the chain break
in the template to line up with your query sequence.