The Employee responsible field on the prospect and customer is constrained by a pre-defined non-removeable filter limiting its values to employees that are/have been employed by the current legal entity only. But the same it allows to choose the employees from other legal entities in vendor master.

Kindly require the same behavior in customer and prospect master. Employee responsible field  should allow to choose employees from other legal entity.

Similarly, other filters like employee type (employee or contractor) and employment status (employed or terminated) are made available for user selection (optional requirement).