I feel this in my bones. Side projects are so cathartic and saved my sanity at $DAYJOB. I don't care that I can't implement things the way I want, or how everything is spaghetti, or how much tech debt has piled up, my side projects is a blissful world that I invented. It gives me the "I am Jack's crap codebase" fight club zen at work.
Yeah. There is something about carving out the image of your own mind and getting absorbed deep inside it. I will write from scratch much more than I need if it strikes me, break whatever rule I want, give names that only make sense to me. It is a sanctuary
You can tell my engagement level with my job by the commit frequency on my side-project.
If I'm deeply engaged at work, I don't have many spare cycles out of hours and there's little happening - library updates and small fiddling.
Otherwise - it's full steam ahead on projects that I somehow magically find the time for.
I don't work on my stuff during work hours - disengagement from work results in more energy and motivation to do stuff out of hours.
Weirdly, I think this actually benefits those boring workplaces too. If I'm scratching the itch with what I'm doing on my side-project - it means I'm less likely to invent interesting new ways to over-complicate things at work.