XML RPC API edit record requires all mandatory fields

XML RPC API edit record requires all mandatory fields

We are using the ZohoCreator XML RPC API to update records in our application. Normally we need to update only single fields in existing records.
In the past it was possible to specify in the XML only those fields, that we want to update, as long as the other mandatory fields already have a value.

Since this week the XML RPC API seems to require the values of ALL mandatory fields, even if the values of those fields don't change. Otherwise the update fails and an error "Enter a value for xxx" is returned.
Is this a bug or on purpose?

Thank you in advance,
Ute