This seems a lot like what my hen Oreo has. She has pendulous crop which when I first got her caused her to eat a lot of food. This is because the way that chickens know when to stop eating is when their crop stretches a little bit so they know it is full. With pendulous crop, the crop is enlarged so they don't realise when to stop eating, as the crop will hold more before becoming full. This often leads to sour crop as their crop is so full that it cannot completely empty overnight. It sounds like you had this problem but that it has now been resolved, however if pendulous crop is the problem here, it is likely to happen again. This is just from my knowledge and I have no experience of your hens breed, but I would say it is not 100% normal for there to be anything at all in the crop in the mornings.
For my hen I have been using a crop bra which was custom made for the size of her crop by a great seller on Etsy. I'm pretty sure this works by restricting the crop so that it does get to a point where it feels 'full' and she stops eating when she should. A crop bra was recommended by my vet too, and she does now eat less and hasn't had any cases of sour crop since we started using the bra - it has been very effective. I was advised by the vet that as long as her crop is at least mostly empty in the mornings and she is not regularly getting sour crop that she should have a normal life, except perhaps a slightly shorter lifespan as it may cause her some more problems as she gets older.
Hope this helps!