LOL
A bug for this was found in 3.12, apparently he moved something about it to "common events" and forgot to do the "callback," causing the egg to not appear to trigger Creepy Crawlies' climax-no-pun-intended-of-course-it-is. He did a fix for it, dunno if it's posted here yet, but apparently the fix causes the egg you want to only appear half the time. Just go back to the dungeon and re-enter the caverns and it will be there. PLUS an extra egg which is there 100% of the time, the one on the left.
Unless the egg you want is always there, just sometimes invisible as you said?