The carbon-12 has less mass than the proton and boron-11 did–That mass became ENERGY, so even though regular carbon-12 is stable, this transitory carbon-12 nucleus has too much energy and breaks apart into three alpha particles.

Wolfram Alpha is a easy tool to look at this.

It actually is possible in a small fraction of pB11 reactions for the energy to be released as a gamma ray instead of in the break-up into alpha particles, and when that happens you are indeed left with a carbon-12. A friend in astrophysics at Caltech suggested it might be interesting to look for the gamma ray signature as part of our diagnostics.

The comments here go into slightly more detail: https://focusfusion.pmhclients.com/index.php/site/article/are_you_sure_pb11_fusion_isnt_fission/