This has probably been asked before on the old bawd so Search shows nothing. Nevertheless.
I have heard American TV people say “We will be back momentarily” meaning “in a moment”. In the UK we would say “shortly”. Momentarily would mean “for a short time” in the UK with the implication of moving swiftly on to something else. “He was momentarily confused” etc.
In the States is this an instance of the meaning evolving or is it just a solecism? cf. enormity and fulsome.