Good point. But I would speculate that something might have happened during the journey to defeat the demon lord that has somehow hidden all that information from everyone, including the Goddess. It might be that she came across some spell/ability/artifact that has the ability to do that during that time.
Or, here's another possibility: she made an unbreakable contract/agreement with the Goddess before her reincarnation that would not allow the Goddess to tell anyone about her background and cheat skill.
But we will be venturing too deep into speculation/overthinking territory if we go down this road so best we let the author cook for now. I too would like to know the actual reason of why her entry is unknown.
more like your second paragraph , goddess made a deal with her to hide kuki's skill to anyone including all incoming / new reincarnators.. thats why her skills listed as ???? in the book..
but i think she will become silkie's ally in killing all reincarnators as she have deep love toward his brother silkie (now a girl)
maybe she will also save silkie from lunatic's revenge after the ring of promise effect gone. Lunatic not exactly happy getting threathened by silkie everytime.
or kuki will order silkie to kill frielee and all orpaned kids
it is possible the end of this manga will be a double team kuki and silkie creating an assasin guild.l