Donkey Kong Country 2 and Open Bus

I have to say as a 6502 assembly programmer I have wasted many hours of my life tracking down the same issue in my code (forgetting to put an # in front of an immediate value and thus accidentally doing a memory access instead). Often it's like this case too where things might accidentally work some of the time.

Worse than the floating-bus in this example is when it depends on uninitialized RAM which is often consistent based on DRAM so the code will always work on your machine/emulator but won't on someone else's machine with different DRAM chips (invariably you catch this at a demoparty when it won't run on the party machine and you only have 15 minutes to fix it before your demo is about to be presented)

Was there ever an architecture that used dynamic memory with a 6502 CPU? In my (limited?) experience, that platform always had static RAM.

Most of them. Static RAM was (and still is) more expensive since it needs more transistors and chip area per bit stored. It it, however, also much easier to interface since it doesn't need refresh circuitry. This is why you see it in the earliest designs, and also why you see it in so many hobbyist designs. It's also why you tend to see it in the video systems even if the rest of the machine uses DRAM. Dealing with DRAM refresh while reading out the whole memory chip sequentially (while also having a second port to read/write from the CPU!) starts making things very complicated.

But still DRAM is what you would use for a "real" system. Wozniak's design for the Apple II used a clever hack where the system actually runs at 2 MHz with an effective CPU rate of 1 MHz. Any read from a DRAM row will refresh the entire row. Approximately every other cycle the video system steps incrementally through memory, refreshing as it goes.

Same with the VIC-II and the 6510 in the Commodore 64. The video chip is given the main character role for the bus, stopping the CPU from moving forward if it needs cycles for video generation or DRAM refresh.