Problem solved! Thank you for nudging me in the right direction.
I used the ProcessMonitor app to inspect the DLLs loaded by both the 32 bit and 64 bit versions of NPP and its plugins during startup, and could see that the difference between them was that the 64 bit version (only) failed to load VCRUNTIME140.DLL which is needed by apps and plugins built with Microsoft Visual Studio.
I had actually installed the redistributables earlier, suspecting this problem. However; the ‘gotcha’ that caught me out is that I had failed to note that Microsoft has separate Microsoft Visual C++ Redistributables for 32 and 64 bit architectures…
So, after installing the 64 bit version of the redistributables (as well as the previously installed 32 bit one), NPP and the NpOpenAI plugin both work as they should!
Interestingly, ‘Dependencies’ didn’t spot this. I may have been using it incorrectly.