Hi there, welcome to BYC!
Also, congrats on your first eggs, that one's lovely!
Possibly yolk from another egg that got eaten?? I mean that's definitely yolk!
Softees or thin shells can be caused by new layer reproductive system (shell gland) coming on line and working out its' hiccups which sometimes become happy accidents that get discovered and enjoyed by a flock member. This is most likely what happened according to my experience, presuming a proper diet with enough calcium (included or free choice on the side) is available on a regular basis.
Sometimes the culprit may have yolk on the head somewhere. I like to keep a fake egg in each nest to deter any curiosity pecks. I have not YET had it actually cause an egg eater of solid eggs.