I find this too! I use coconut oil to lubricate sometimes, just thought I had used too much. But good to know others experience it too.
Don’t wash just boil in the water
It’s fat residue from breastmilk. You can use a soap specially formulated to breakdown breastmilk fats like dapple baby bottle soap (at target)
What helped for me is never ever leave them dirty. As in have a thermos there w hot water and as soon as a bottle is finished pour an inch of hot water into it, shake it up and then that way it’s “rinsed”, until you can get to it and actually wash it. We always had a hot thermos there I made one up every single morning w the first boiled kettle of the day. Any overnight bottles used are already “rinsed” before we gather them and take them to the kitchen in the morning it’s so much easier to wash them coz there’s never any residue
I use dawn powerwash it works so good and the bottles come out really clean and don’t have any scent
Stick them in the dish washer. That will get the grease off. You will still need to sterilise them afterwards.
I notice that happens when the bottles are breastmilk. If it's formula, I find it doesn't happen.