Several states have issued Advisory Opinions to the effect that as long as the lawyer is not "hanging out a shingle" in the state where they are not licensed, but are working remotely on matters for existing clients in another state, the unauthorized practice of law rules are not implicated.
The State Bar of Florida has addressed the issue specifically in the context of patent and trademark lawyers who are practicing "Federal law." According to the Bar’s notice:
“The UPL Standing Committee, pursuant to R. Regulating Fla. Bar 10-9.1(f)(3), provides this notice of filing a proposed advisory opinion. The proposed opinion finds that an out-of-state licensed attorney working remotely from his Florida home for his out-of-state law firm on federal intellectual property rights matters (and not Florida law) and without having or creating a public presence or profile in Florida as an attorney would not be engaged in the unlicensed practice of law.”