I am changing the Fd2 property of the Table1 association that is in the Table2 entity.
the problem is that it's updating all the properties of Table1.
the Fd21 is getting empty because it is not in the json.
Can you please give more details about the operation you are doing? How are you performing such patch? Using TXDataClient? Using a different library? Using a tool?
Remember that you must use HTTP PATCH method (not PUT) to achieve what you want. When using PUT, you have to send the whole object.
The entity must not contain associated entities as inline content. If values for navigation properties should be specified, then they must be represented as an association reference. For single-valued (single entity) navigation properties, this will update the relationship (set the value of property to reference the associated object represented in the association reference).