Invisible gaps

I have an issue that recently came up with some of my student prints. When you first load the model onto Cura, it appears to be a solid print, but when you start printing, there are gaps and holes that appear. Upon further inspection, I realized that in the “Layers” view, the gaps and holes are there. Any ideas of how to fix this?

This came up in a project where we converted line drawing jpg files into stl files through Tinkercad. To fix it, I usually had the kids redo it by making the lines in their drawing a little thicker. It seemed to have worked, but there must be another way to fix it through Cura…any suggestions are welcome!