If anyone is working with Advanced Manufacturing/Mobile Manufacturing, if you set a Work Center on the Work Order, core NetSuite is changing BOM Quantity and Quantity on the line level which it shouldn't be doing. We finally got NetSuite to log a defect for it (U6 somehow... should be way higher IMO):

Defect 631687 :  User Event > Work Order > Setting value for "Work Center" via script setValue() causes the line item "quantity" and "bomquantity" fields to source their values from BOM Revision

Could thus also result in a negative quantity on the work order?!?! It's been happening to me recently and I can't figure out the source of it.

I'm not sure what the calculation does exactly. If you're setting a Work Center yourself via setValue(), then Its possible i suppose.