No memory released when closing Thunderbird in Windows

[PeterA23]

I am closely watching the Parallels Resource Monitor:

When closing Mozilla Thunderbird (which is particularly resource-hungry) inside my Windows 10 VM, no memory is released from the displayed 18.3 GB! Could this be a bug in the Parallels Resource Monitor, in Parallels, or in Windows itself?

 

[PeterA23]

Windows doesn't seem to be the culprit because, BEFORE closing Thunderbird, Window TaskManager shows:

...and right AFTER closing Thunderbird it shows:

So there IS a diminution in memory. So it seems that Parallels Resource Monitor or Parallels is the culprit!
Also, Parallels Resource Monitor should show TWO digits after the decimal point! That would get a more exact picture of the memory consumption inside the VM!