How to disable bridging
When printing in dual colors, in this case to engrave letters and numbers, Prusa Slic3r adds bridging on top of the printed layer. Its not necessary since there is no void space to cover with bridges.
The first layer is printed in two colors
The second layer and other layers is printed with single color. Note the bridging over the numbers.
RE: How to disable bridging
I suspect it may be your model is at fault. Some times when you cut something out of a block in a model the cut will some times leave an invisible plane that the slicer is trying to bridge. If you created the model, go back and make the letters slightly taller so that they extend above the block and then do the cut.
If you do not have access to the model to make these changes you could also try changing the layer height you're slicing at. A slightly different layer height may "skip" the invisible layer.
RE: How to disable bridging
@darcshadow
The model is actually a composite of two STL files, one for each color. I did have som problems with aligning the two STL files, so maybe a slight mismatch in height might have caused the bridging.
This model was made using openscad, but I might try to make another test case using FreeCAD. Unfortunately STL files do not carry color information, at least not in a standardised way.
Which 3D file formats can describe multi material which is understood by Slic3r?
RE: How to disable bridging
Solved the problem. After reading https://forum.prusa3d.com/forum/prusaslicer/manual-multicolor/ I added both STL files at the same time. When adding two files, a prompt asks if it is a multi color object, and answering yes will result in an object that when sliced will not have the bridges, only solid layers.