Hi,
When you use the 'parallel hints', does this actually then show you a parallel query plan or is it being ignored? You may well need to enable parallel at server-level, 'max parallel degree' etc to make successful use of a forced parallel plan. You could also try turning on 'enable deferred parallel'=1.
This would allow the optimizer to consider a parallel version of the best serial plan it decides on.
Short of that, I couldn't really offer an explanation as to what the 15.7 plan is doing compared to the 12.5.4 one without seeing the full showplan\set statistics output.
Cheers,
Simon