I found an interesting issue, not sure if someone has already found it before.
Upon killing the Devourer of Gods, the pumpkin moon and frost moon get buffed, with enemies gaining an increase in health and attack. However, it seems that this "buff" affects anything that spawns during the timeframe, as I discovered when I accidentally summoned the DoG during the frost moon event. His health raised to a whopping 7200000HP (normal mode) when it should have been 600000HP. His attack on the other hand, remained the same. Note that even in death mode, the DoG's health can only be up to 1360000HP. Note that no other mods were used (other than this boss health mod and No NPCs dead mod)
This, I believe, virtually works with any boss as long as they were summoned during the duration of a frost/pumpkin moon, as I replicated it with Calamitas and even the Moon Lord as shown below:
So for anyone who wants a long drawn out battle against some bosses, be sure to summon them during the pumpkin/frost moon event! If you don't want the event interfering with the boss battle, use a cosmolight to quickly push the time to the next night (day also works, but some bosses despawn during the day).
Fun fact: Comparing these numbers to the actual boss health numbers, we can conclude that every boss's health will be multiplied by 12.
Last note: This issue ONLY increases the boss health of the current phase. So let's say u have DoG at 7.2m health and u kill him, his next phase will spawn his normal health if the frost/pumpkin moon events are not active (but if they are active, he will spawn with inflated health). Loot is NOT affected, u will get the standard loot and not inflated loot.