Edit Access of Account Team Members Differs Between Lightning Experience and Salesforce Classic

Issue: Account Team Member Access Options Differ Between Lightning and Classic

When editing an Account Team Member's access in Lightning Experience, users may notice that the available access options differ from those shown in Salesforce Classic. For example, in Classic a user may see the option to grant Read/Write access to an Account, while in Lightning they only see the option to grant Read Only access.
Consider the following example scenario:

• An org's OWD for Account is set to Private.
• The org has two roles: CEO and CTO.
• The CTO role reports to the CEO role.
• A user with the CTO role creates an Account record.
• A user with the CEO role creates a child Contact for the Account.
• The CEO clicks the New button within the Account Teams related list.
• At this point, they are only presented with the option to grant Read Only access to the Account.

Similarly, users may see an option to grant greater access in Lightning than in Classic. If they try to add the Account Team Member with greater access, they receive the following error:

"insufficient access rights on cross-reference id"