Square D Panelboards accept both QOB (bolt on) and QO (plug on) breakers. I have been told that they recommend using QO breakers instead of QOB when adding additional breakers in the field. I can pretty much guarantee that the panelboard was not listed for tandem breakers and I’m sure the breaker used was clearly marked non-CTL for replacement use only.
BigB, what new Homline tandem breakers are you referring to? HOMT breakers in 15 and 20 amp have been available since the beginning of Homeline. They later added the larger quad breakers (30-50).