Hello Mark_Sealey!
In general, I don't recommend exporting your passwords since that will remove them from the secure and encrypted environment of 1Password. Any exported copy of your passwords will be in plain text and so will be readable by anyone who views it. In addition to this if you change any of your passwords in the future the exported copy will become outdated.
Rather than exporting your items then might be a better way to accomplish your goal. Let me know why you're trying to export your items and I would be happy to help.
When I do a File > Export of my vault, 1P 8 (which I love, thank you!) defaults to the Downloads folder. 1P 7 used to remember the location I chose, which was actually on an external volume, also backed up by Time Machine).
I've reproduced this behaviour on my end and you're right that it differs from the export behaviour in 1Password 7 for Mac. I've filed an internal work item so that our developers can look into this further. Thank you for reporting the issue!
-Dave
ref: dev/core/core#15564