I hope this is not some stupid "game mechanics" but really a bug.
When a building is normally destroyed during construction, nothing will be left of it. However, if we give the order to repair or "support", after destruction wreck apear allowing player to build from 50% of the total hp of the building (even if destroyed buidling had 51%). This leads to a bizarre situation, where the constant bombing of the building ceases to cause any damage to it, because you can still start construction from 50%. I recorded a video where destroing 3 times <50% yolona actualy do nothing. Only when i stoped giving repair order, bombers could finaly destroy it.
https://www.youtube.com/watch?v=YjAyUNt-fJ8