I created a simple test model with gaps in the bottom and separate parts for rings at the top so that I could test printing multiple colors, and fine tune small bridges without supports at the same time. Here it is from the top and bottom, multicolored to show the different rings:
For some reason, the first layer is sliced incorrectly when the parts are set to multiple colors.
With all parts set to the same color:
First layer is fine:
It’s also fine if they are all set to white.
Parts set to different colors:
First layer is suddenly missing a quadrant. Second layer (not pictured) partially adds it.
Here are the parts separated out and viewed from the bottom, in case that helps:
Does anyone have any idea what’s going on here?