Outlook for mac 16.20 - import .olm file then get an empty "Outlook for Mac Archive OLM" folder

I'd switch to a new Macbook Pro in near future so would like to test the export/import function on my existing Macbook. After I imported the .olm file (completed as the import dialog showed) that I exported on the same computer, I got an empty "Outlook for Mac Archive OLM" folder in "On My Computer" folder.

The .olm file is 2.5GB and the import process did take a while to complete so there should be something in the "Outlook for Mac Archive OLM" folder but it shows nothing in that folder.

Anybody know why?