Rubber egg shell.....why?

My second chicken to give us eggs laid her first today. It had a very thin brown rubbery membrane and had splat on the coop floor. At first I thought it was a weird poop but upon closer inspection could see what must have been the "shell" and a yellow yolk color in the slimy goop. I hurried and cleaned it up as the other chickens were looking it over and pecking at it when I found it. My Question is why is the shell like this? Her sister that has been laying an egg a day for the last 9 days (first eggs) have nice hard shells on them. Does this mean the second chicken that laid the rubber egg isn't getting enough minerals in her diet or is this just a fluke that has to do with it being her first egg? Poor girl keeps singing the egg song on and off and then going back to the nesting box to sit. I think she is very confused as to what just happened and has no egg to fawn over. Is this something I need to worry about or is this a common first egg laying thing?

*Chickens Diet- All my girls have been on layer mash since 18 weeks old. They are now 21 weeks old. They do get to free range 2-8 hours a day depending on if I am working or not. They also get a lot of leftover scraps from the kitchen and garden.

Thanks for any insight you can give me.

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That's totally normal and very common for a first-time egg layer. They'll normalize, not to worry.

It's a good idea to have oyster shell available to them free-choice though.

Here's a photo of my first "rubber egg". You can see from where my thumb is how pliable the egg was. She never laid another like it. Others have laid eggs with paper-like shells or no shells at all. It's all normal. BTW: she is an Easter Egger and her eggs are a greenish blue.

Yep! Completely normal for new layers! Their body just has to get the timing down better. The egg passes through the body in stages. As it moves down the system the shell is deposited around the yolk. Her body just has to adjust its timing to allow the shell to fully form/harden. It will get better!

Congrats on the new layer!
God Bless!

Thanks for the replies. That makes me feel much better about it. I was worried I had caused it by letting the girls free range so much. I think I will go purchase some oyster shell to put out for them as well just to be safe.