Hello,
After about one year of running into this issue intermittently I have finally figured it out. This is not a bug (in the sense that there is no code error or anything of the like) but rather a hidden behavior when importing DXFs and dealing with layers/styles.

I can now repro it because it has just happened to me while importing a DXF created by another person. To repro, get one of the DXF files from this project, like the Wood Control Box :

https://www.thingiverse.com/thing:797666/files

It opens up and imports just as expected. But, not only does it import the drawing, it also imports the following styles :



This of course is not necessarily an issue in and of itself. But if one decides to open this file rather than import it, then the default MOI styles will not be present in the current work scene. So if someone then saves this as .3dm the scene will not have the default MOI style colors that a MOI user may be familiar with and may rely on for organization. The reason why I didn't catch this for the longest time is because I do not have the Styles panel visible in my UI since I very rarely need to interact with it (I use buttons to send parts directly to a given layer/style and hide/show the few layers I always use for organization : default black, red, and blue).

So in a way there is nothing wrong under the hood and the workaround is easy : ctrl-c the parts, and ctrl-v them into a fresh MOI scene that has all the styles. But perhaps it would be good to have some way to re-generate the default styles directly from within the interface, since one may consider these colored styles as a core component of the MOI workflow.