Jup, and if your employer doesn't stop you or even expects such behavior that is called a bad work culture.
Sure, I would also do long programming sessions on my own things every now and then, but those happen because I am in the flow and it feels like more work to come back into it the other day. But those are my own projects. I don't owe this to my employer. If they want that they could offer me double my current salary and even then I would insist on only doing it when it makes sense to me.
In my case my superior comes by in my office or sees I made a commit and tells me I should stop working (I tend to forget to look at the time if I am in a flow).
But I also live in Europe and my superiors want me to come in with a fresh mind, instead of burning myself for things that should be normal work.
Sure, I would also do long programming sessions on my own things every now and then, but those happen because I am in the flow and it feels like more work to come back into it the other day. But those are my own projects. I don't owe this to my employer. If they want that they could offer me double my current salary and even then I would insist on only doing it when it makes sense to me.