I haven’t used virtualbox in a long time so I can’t reproduce this.
Are you sure gateway is ok? Next time you experience this, run whonixcheck in the gateway, and then after that, run it again in the workstation.
Could the gateway VM be entering suspend mode?
Are your activities in the Workstation modifying its firewall in any way?
Do you have applications configured to use port 9052?
Do you have slow hardware or is your system under stress when this happens?
We can get more detailed information by running:
bash -x whonixcheck --function check_tor_socks_port_reachability
bash -x whonixcheck --function check_tor_bootstrap
BUT don’t paste results without sanitizing (your VM names might appear in the output).