There is no bug people. You cannot create large commercial districts without having the proper population. Look at my screenshots, I have a population of 20k and my demand for commercial is pretty high(50%?). Could I build a large commercial downtown if I wanted? Yes, but there needs to be the proper ratio of offices, commercial and industrial parks, along with residents to fill those jobs.
Zones
Parks
Traffic
Downtown/Commercial demand
My computer isn't the best for visuals in this game because it's not really a gaming computer, but this proves that there's no bug. People need to learn to balance demands and not build a huge METROPOLIS right off the bat. Do you really think you're going to have huge skyscrapers, a bustling downtown and large commercial demand with a population of less than 50k? Expand slower and create low density for smaller houses and high density for your downtowns, but don't try and build a metropolis with only 6k population lol. You people are way too demanding of a company. CO/Paradox have created an amazing game that they'll be developing/evolving as time goes on, but don't demand a fix when you don't know how to build a city.
Since I play city builder since the ever first SimCity, I think I know how to build a city. And it's totally unlogically that parks kill off commercial demand. I can build a full city without any store. Don't get me wrong, I don't need skyscrapers for commercial while I have some 10k citizens. On the oher hand, while I see your Downtown, you have offices and not commercial area signed. Offices are handled like industry! So don't call me that I don't know how the game works, dude!