This issue is the legacy “Split floor to thermal zones” component. It just has some really poorly-written parts of it that were done by yours truly. If you were to connect the output of that component to the newer LBT plugin components for creating zones, you’ll see that the geometry is actually curved, resulting in some intense meshing when it goes off to EnergyPlus:
I suggest either not using the legacy “Split floor to thermal zones” component, or if you want core-perimeter offsetting that’s reliable, then use the dragonfly components in the new LBT 1.1 plugin. They are much more reliable thanks to some straight skeleton methods added by @SaeranVasanthakumar :
