At Ignite we announced a major improvement to the way secure external sharing of files and folders works in both OneDrive and SharePoint in Office 365 and we wanted to share what this means for users...
Updated Jun 25, 2020
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hwelvaar this change must have happened because the your users became part of their tenant (Azure AD B2B). I would suggest looking into the foreign Azure AD and check for guests. The people affected will be registered as a guest. SharePoint will switch from SharePoint PIN sharing to Azure AD B2B if the account is registered as a guest. This will happen if the users are part of an Office 365 Group (or MS Team). Groups rely on AAD B2B and will register guests in your tenant. You can prevent (end-)user from registering guests through AAD configuration, but this will limit Groups guest membership and also other services could be impacted (AIP, ...). To get back to PIN sharing they could delete the guest accounts in their directory, but this should be tested if and where they will lose access based on for example group membership.
What is the reason you don't want them to use their AAD account? Conditional access rules?
Ciao Marco