Media Center 26.0.107 64-bit MacOS 10.15.6
This is not a new issue with v26 - observed in previous versions as well.
The issue occurs on every attempt to resize/drag. I have observed it on a MacBook Pro 13, MacBook 12 and Mac Mini using a variety of different monitors including the built-in laptop displays.
When you mouse onto a Mini View window and then attempt to drag it to a new location, the window grows in x-y dimensions until you release the mouse button. I have used a variety of Logitech mice including MX Master 2 and 3 and MX Anywhere 2S and an Apple Magic Trackpad 2 - behavior the same regardless. Subsequently, it appears you are only able to resize the window from the top and right edges, although getting a resize handle is a laborious and hit-or-miss affair, especially when attempting to resize from the right edge. So, upon resize back to the desired dimensions, the window is pushed downward on the screen. When you then attempt to drag it back up to the desired position, it all begins anew. Thus, getting a Mini View window to be the desired size and position on the screen is exceedingly laborious and difficult and/or impossible to accomplish. (Of course, this does not occur in the Windows version.) Also, this does not occur when using Standard View although Standard View behavior is problematic for other reasons including having the window jump around on the screen when you move it and also great difficulty obtaining and using resize handles.