Hello Sean Faherty,
Welcome to Microsoft Q&A forum.
Based on what you described, the issue usually happens when you restart the slave PC (and sometime even when shutdown). This could come from the settings of the apps, when the physical mouse is not present, the apps might need to draw a cursor from the software itself.
Before we go further, please try this quick fix.
On the slave PC, go to Settings > Accessibility > Mouse and turn on Mouse keys. It will make Windows think the mouse is a point device and prevent switching the cursor when the physical mouse is not present.
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