The issue here is that the API does not support Cr
oss Tenancy in the case where the same drop down value exists in multiple tenants.
When you are inserting into Tenant A, the API will utilize the drop down value from Tenant A.
I will create a feature request for an update to this logic - but I do not expect to see this change to be implemented anytime soon. The work around here is to make the duplicate value unique across tenants OR authenticate with the API using the Tenant for which a record is to be created.