I'm working on a SharePoint 2019 farm where I've done a migration from 2013. Users come from two domains in separate AD forests. The issue I'm having is that the User Profile Service only syncs users from the same domain the SharePoint servers are in. Both sync connections are set up, and users show up in the containers when creating the connections. The farm account has replicate directory changes on both domains, and we're using separate user accounts to access the domains.
Additionally, if you are doing anything that uses the people picker, Active directory shows 0 users from either of the domains.