Offboarding mailboxes fails with “PropTagToPropertyDefinitionConversionException.”

Hybrid M365 setup, just recently upgraded the on-prem server from Exchange 2019 to Exchange SE.  After doing so, migrations from Exchange Online back to Exchange On-prem fail at 10% with the error “PropTagToPropertyDefinitionConversionException.”  

I opened a case with M365 exchange support, and after some time, they came back to tell me that the Exchange Online portion of the process is not at fault, and that I have to engage the on-premise support team (this seems a little nuts to me, as its all connected and all supported, but I've been in this business for 30 years now, and it's not the first time I've seen buck-passing), and/or ask this community for help.

Hence, this post.

That error appears exactly two places on the internet, as far as I can tell: a blog (in German) from an Exchange expert doing cross-tenant migrations, and a page at 

https://west.jcteams.info/bhit11/docs/EX1232513.html that seems to describe my exact issue.  Neither had useful suggestions - mostly, they say this:

Set-MoveRequest -Identity "<UserPrincipalName>" -SkipMoving FolderRestrictions
Resume-MoveRequest -Identity "<UserPrincipalName>"

That didn't actually work, but when I tried the same parameters with Set-MigrationBatch, they worked as long as I ignored the message "The SkipMoving parameter is deprecated. Use the MoveOptions parameter instead. If you have any scripts that use the SkipMoving parameter, update them to use the MoveOptions parameter."  

So what was a simple process is now a more cumbersome workaround.  Does anyone have an idea on how to troubleshoot "PropTagToPropertyDefinitionConversionException?"