I'm converting a reeeeaaally old 1.0 scheduled script that I'd inherited to a map reduce, so some of this isn't the way I'd do it if done from scratch and will probably clean it up a bit.

In my experience you need to reassign the filterExpression object for it to work properly.

const filterExprCopy = mrr_np.filterExpression;
// modify your copy using push etc;
mrr_np.filterExpression = filterExprCopy; // now it will use your new filters

This is because, last I checked, NetSuite implemented filterExpression as a getter / setter of an internal private array, rather than direct access to it. The getter produces a copy of that array. So if you just do

mrr_np.filterExpression.push(...)

that is only working on the copy and not the internal array.

I'll try that, thanks