If you're using a Microsoft account and not an Azure AD account, you can run into this issue. Can you confirm that you are using an Azure AD created account when this happens?
AD Connect prerequisites:
An Azure AD Global Administrator account for the Azure AD directory you wish to integrate with. This must be a school or organization account and cannot be a Microsoft account.
This list is starting to be quite long so I understand that it is easy to miss. As soon as I pick something up in these forums, I add it to the documentation.
It is also possible that there is a connectivity issue or duplicate object. https://learn.microsoft.com/en-us/azure/active-directory/hybrid/tshoot-connect-connectivity
Also, check your conditional access policies to make sure there isn't something blocking unjoined devices.
We are fairly new to Azure so just leaning though we have all used AD.
Users can sign into https://portal.office.com/
With regards to Sync methods I am not sure what methods there are TBH. We are doing a delta sync and using Password hash sync(PHS) plus Seamless single sign-on hybrid authenticaion.