Tenant to tenant migration using Outlook export/import only imports content from previous year

[optimystery]

I am attempting to do a tenant to tenant migration from 2 Office365 accounts. Microsoft 365 support has not been able to offer any help on this, even though I'm trying to escalate to a manager.

I am using the export/import method to .pst on Windows (running in Bootcamp). For some reason the .pst file is not being fully imported. I can see the 2 accounts show very different disk usage after import. The original is roughly twice the size at around 14gb.

On further investigation, I found that all emails older than 1 year (to the exact day and time of the export) are blank. They contain headers, but not content or attachments.

There is a setting to choose how much email to download, and it's set to all for both accounts.

3rd party migration tools are apparently for bulk migrations (I only need to do 2 mailboxes). Mover.io apparently used to do this, but was bought by Microsoft and no longer works (I queried their support).

Powershell-based methods seem to be very poorly documented.

Over on superuser forum, I was suggested to:

Disable "Cached Mode" prior to exporting your PST, this will cause Outlook to pull data directly from the Exchange server which will take a little bit longer but will give you a complete mailbox export.

I have done this. However:

The .pst file exported with cached mode on, but download emails set to all, is 13.8gb. The completed .pst using cached mode off is 13.4gb. Not sure why this difference in size? Which one of these is "complete". Why does the import not bring in all of this content?

After importing into the target account/mailbox previously the size ended up being about half of that.

It seems that the export is doing ok, but the import is not. It seems very odd to me that this kind of work cannot be done in the cloud. Or have I missed something? As I mentioned, thus far none of the engineers has been able to explain why this is happening, or suggest steps or alternative methods.

In case anyone comes across this problem looking for a solution, there is an open post here, which may ultimately lead to an answer:
https://superuser.com/questions/1577252/tenant-to-tenant-migration-using-outlook-only-imports-conten...

[#2]

Hello
I would suggest the AZCopy Method. It has worked flawless for me so i can tell you by experience , it has that extra mile , make the export, import to the Blob , map the pst to a CSV file but for 2 accounts it should be very very easy. Do you have a good Internet access ? if you do don't waste your time!
https://docs.microsoft.com/en-us/microsoft-365/compliance/use-network-upload-to-import-pst-files?vie...