Random idea, with no evidence, are you sure you selected the A250 in the settings? In Luban, click on the gear icon in the lower left corner, then Settings > Machine Settings. The Machine drop down should be “Snapmaker 2.0 A250”.
Although, that idea would only affect printing in the corner, it shouldn’t affect the size…
Maybe try some of the pre-sliced models in section 4 of the 3D Print Guide. I have a temp tower handy, so let’s try that. Download the 190-225ºC temperature tower for your model, and print it. Mine was 8cm tall.
If the pre-sliced GCode prints correctly, it indicates a Luban config of some sort. If it does not print correctly, it’s more likely a controller/hardware problem.