I have an extremely simple User Event script that modifies a custom field using the before submit event.

function beforeSubmit(context){
            const purchprice = context.newRecord.getValue('cost');
            const purchconversionrate = context.newRecord.getValue("purchaseconversionrate")
            const priceum = Math.round(purchprice / purchconversionrate)
            log.debug("priceum",priceum)
            context.newRecord.setValue({
                fieldId: "custitempriceperconsumptionum",
                value: priceum
            })
            log.debug("rec",context.newRecord)
      }

I can see it's running and the second log returns the record with the value set but the changes aren't being saved on the item.

Is there something about the definition of your field that is overwriting whatever you are doing here?

Ah. I had the field set to view only. My bad.