After seeing ne0n post on the discord a screenshot of the Ancient Golden Bug receiving abnormal damage, I decided to test it myself. What I found out is, after you kill all 3 eggs at any point (before damaging or in the middle of the fight) the Ancient Golden Bug starts to receive massive damage (probably due to his mdef/def changing to negative values?). Another thing I noticed as well is, if you let the Ancient Golden Bug absorb his eggs BEFORE receiving any damage, his status rises and starts dealing more damage (one egg absorbed quadrupled his damage).
I forgot to take SS on the quadrupled damage, and the source is the image from ne0n's perspective.
Please check both behaviors on hard mode as well, as i'm not able to test it myself.
TLDR: Easy Mode Ancient Golden Bug's spawned eggs are changing his stats, increasing exponentially if absorbed at full health, massively decreasing if all 3 eggs are killed at any point.