I have created virtual machines for:

Ubuntu 18.10
Linux Mint 19.1
Elementary 5.0
Zorin 12.4

on both Windows 10 and Zorin 12.4.

I installed the latest Guest Additions and run "sudo apt-get update" then "sudo apt-get upgrade' in terminal on all the virtual machines. Extension pack was also installed.

I then ran experiments resizing each virtual machine window several times in quick succession.

I had no issues resizing the virtual machine windows for any of the virtual machines except for Zorin. If I resized the Zorin virtual machine window several times in quick succession I lost the desktop. I used the recording function on Virtualbox 6 to record this, it illustrates the problem quite well. I am not able to upload it here due to its 1.3 MB size but can provide it to anyone that may be interested. Because this issue was only with the Zorin virtual machine I have concluded the issue is with Zorin rather than Virtualbox.

I am wondering if anyone else has had this problem?

would be nice to know how did you exactly resize. What kind of desktop is it? What do you mean by lost?