B2B External Members -access to Private or Shared Channels

Hello Paul - thanks for responding - I'll try to clarify!
Yes, they do get to the tenant as expected and are able to access SPO sites and Teams they've been added to.
A bit more history: we have mutliple tenants. We've recently configured Cross-Tenant Sync [https://learn.microsoft.com/en-us/entra/identity/multi-tenant-organizations/cross-tenant-synchronization-overview] and the provisioning/deprovisioning of 'External Member' accounts in our tenant is working just as expected - it's been a huge benefit in keeping user information from our partner tenants current, GAL visibility, and more. The synced users are not Guests, nor are they standard Members - they are External Members [very specific tenant UPN syntax].
A significant driver in our adoption of Cross-Tenant Sync was the automated provisioning/deprovisioning of these users, such that we'd be able to add them to Teams and SharePoint sites within our tenant; we ARE able to see/add the synced users to Teams as MEMBERS, just as expected. However, their experience once added to a Team is not what we expected.
Even when directly added as a Member to a Private Channel, they are unable to see/access the Private Channel; similarly, they are unable to see/access Shared Channels that are configured to 'Share this channel with everyone on the team'
What they can see/access is only the default General [Standard] channel, and any other Standard channels that are created within the Team.
As Members (albeit External ones) - we fully expected that they'd be able to participate in both Private and Shared-with-everyone-on-the-Team channels within our tenant.
Hope this is at least a bit more clear - it's a pretty specific use case.