Rick Christopherson
They added the two pole breaker because it is required by code. When they added the new panel, all work had be be updated to current code at that time. That doesn't mean that the whole house had to be brought current, but any aspect related to the load center did.

You have a Multi-Wire Branch Circuit (MWBC), and at the time it was created, code did not require a 2-pole breaker unless it also served 240 volt loads. Code was changed so that all MWBC must use 2-pole breakers regardless whether they serve 240 volt loads or not.