Hi, my name is Neil, I'm an independent adviser and a Windows 10 user like you.
Sorry you're experiencing this issue in Paint 3D.
Please try and reset the App using the following instructions:
Press your windows key + I to open Settings and then navigate to Apps > Apps & Features > Paint 3D > Advanced Options > Reset and then press the Reset button.
This will reset the App and hopefully resolve your problem.
Let me know how you get on.