This stems from Is the Spell Bombardment additional damage die subject to critical hit dice doubling?
I re-read the description of Spell Bombardment
When you roll damage for a spell and roll the highest number possible on any of the dice, choose one of those dice, roll it again and add that roll to the damage.
and noticed that the wording is different than other features adding damage dice, which usually say "extra".
Does the Spell Bombardment's die count as damage die? Or it is counted as a modifier, because the wording says "add that roll to the damage"?