how to use a debugger

Exactly!

In the openMSX debugger, the easy way to do it is this:
- start the debugger and connect it to openMSX
- hit the "break" button at least once (the red circle): this is needed, as for some reason, if you don't first break openMSX, some times (about 50% in my case) any breakpoints that you set will not be actually set. You only need to break once, then you can let it continue running if you want (the "running dude" button).
- in the "breakpoint" menu at the top, click "Add ..."
- then select the "Memory write watchpoint" type, and specify the address or range you want to detect
- there you go, let your game run, and the debugger will stop as soon as an instruction writes there!

Something that is amazing in openMSX is that once you are in a breakpoint, you can actually "step back in time", to see what happened earlier that led to the state you are in. If you want to go back step by step click the "step back" button in the debugger. If you want to go back longer intervals (e.g., a whole frame, or 10 seconds), then it's easier to do it from the console of openMSX itself, for example: "rewind goback 0.1" will go back 0.1 seconds in time).

openMSX is amazing for debugging. So, I'm sure you'll find the issue

You can also use the openMSX console (F10) an add watch or break points from there (with the debugger attached, of course).
For example:
debug set_watchpoint read_mem 0x1234
debug set_watchpoint read_io 0x2E

Small correction: reverse goback 0.1

There’s also prev_frame and next_frame which I use quite regularly.

Wohoo!! Nice! It's an extremely useful tool!