NX12 and earlier with the assembly as the displayed part, a component the work part, create a datum plane on the solid body from another component within the assembly.
In NX1847 and later when selecting a face of the other non-workpart, that component then becomes the work part automatically and the datum is created in that part instead.
Solution
Added in NX1847 and later releases is the option:
'File -> Utilities -> Customer Defaults -> Modeling -> General (tab) -> Allow Automatic Work Part Change'
Change the default setting by unchecking it to work like NX12 and earlier releases of NX.
It is located at the bottom of the dialog (see image below).
NX11 and earlier Customer Defaults dialog
NX1847 and later Customer Defaults dialog
Hardware/Software Configuration
Platform: INTL64
OS: windows
OS Version: 10_1903
Product: NX
Application: GATEWAY
Version: V1847
Function: CUSTOMER_DEFS
Ref: 002-8015067