My understanding is that NG eventually wants to shift all of their engineering (and manufacturing, possibly?) to Melbourne. Haven't seen this as a publicly announced strategy, but I was told so by a higher level manager working at NG (grain of salt). but it makes sense since there have been issues with engineering/manufacturing being on the same page when they are divided between Melbourne and Palmdale. There's also precedent for them consolidating their programs in Melbourne, as they moved their entire E-2 program from Bethpage, NY to Melbourne.
With that being said, the new facility is probably a partially planned expansion to have more room for engineers, but also a gamble in the hopes that they win the contract. Ramp-up in staffing is one of the bigger concerns of the Navy/GOV given how much the B-21/F-47 absorbs the key workforce, along with other projects in the works.
Boeing's expansion in St. Louis was equally a risk, but it was also needed in order to be competitive for future contracts (including F-47).