Hi there
I would join this query as I'm facing same issue with setting up ADD connect. I work to setup sync between on-prem AD and o365 with same set of accounts. I went trough AAD Connect custom setup carefully following documentation and set up sync between them. I have customer default domain in AAD as well as same domain suffix added in on-prem ad. However It seems to be not working as expected.
When I create new user with custom domain UPN it is synchronized to AAD with custom domain UPN, but after moments it switch back to default domain in AAD as well as in .local domain name in on-prem AD.
Can you advice what may be causing this?