I think the theory that commercials leveling up causes this is correct. I haven't had to zone commercial for ages. Just kept growing slowly, and now occasionally a new one shows up. It starts leveling up immediately (due to good services and land value) and instantly drains the demand.
If you want large commercial districts, I think you have to just grow really big, or get a large amount of tourists.
Just tested this out, I was using quite a bit of smaller parks from the workshop as well as some plopable monuments. After I demolished my parks commercial demand went from "no demand" and increased dramatically, quickly developing commercial zones I had laid out at around 20k pop (at 50k now).
Since this issue doesn't seem to be affecting everyone I wonder if there is a specific asset that is causing this bug.
I'm wondering if it's some of the special parks? I tend to use the Paradox Plaza and Bouncy Castle fairly liberally. It also could be that some of us just build more parks in general.
Well, since the game is broken tonight I may try making a slum city where I don't build any parks or education. Should be interesting.
Interesting because I use some parks I made, which I haven't distributed. These are things like 1 x 1 squares like we used to have in the old SimCity to put down median strips between wide avenues, etc. These too seem to affect the demand.
This could be a bug of sorts, now that I'm watching the reaction to the parks. It could be that the need value for higher-end stores isn't low enough as we upgrade the property values with parks compared to that for the lower value property. This means we would need to build a gazillion high-end houses before we see a tiny bit of growth in commercial, which is what I think we're seeing now.
So it might not be leisure lowering the demand itself but having too many high level residential that is decreasing commercial demand.