How to Split STL Properly for Printing

I have an STL that I split into two pieces with Meshmixer. I will be doing this for dual color printing.

The problem I run into is it makes the part hollow as it removes the face in the process. Is there some way to do this better or fill the hollow part after splitting the STL?

Attachment 11248

This is an example of what I'm seeing. It seems like this should be pretty easy to aoid and/or fix, but I can't figure it out.

Thanks!

When you use plane cut, be sure to change the setting to close the face. The setting I use most is remeshed fill.

Thanks for the response. That would work on certain occasions, but I don't always need to do a plane cut.

Like on this example I posted in the image. What I did was split just part of the image and it wasn't really a plane cut. Is this possible without it missing the face like that?

I have figured out how to do this from a couple YouTube tutorials, but they don't explain how to do this if the face group has a blank space in it. For instance the number 0 or a circle.