"more frequently been" is correct and usable in written English.
You can use it when you're talking about something that has been happening regularly or with increasing frequency. For example, "The number of students taking online classes has more frequently been increasing over the last few years."
Due to difficulties associated with expression and crystallization of full-length IGFBP-5, fragments have more frequently been studied.
Although Mr. Faye talks about the close connection between Heidegger and current right-wing extremist politics, left-wing intellectuals have more frequently been inspired by his ideas.
The acid used to be made from citrus fruits, but since the 1920s, it has more frequently been made through the fermentation of mold.
Guernica's viewers have more frequently been - as Gijs van Hensbergen puts it - "dumbstruck and mesmerised by the power and scale of the image as they stare wide-eyed at the painful drama acted out before them".
Although this rule has more frequently been violated when it comes to vice-presidential picks, there is evidence that presidential candidates who have more "extreme" ideologies (closer to the left wing or the right wing than the electoral center) underperform relative to the economic fundamentals.
To the extent that they have erred, they have more frequently been (contrary to popular myth) slightly on the side of fiscal optimism.
More frequent painful crises and visiting the unit more frequently were positively associated with depression.
Our data contradict the widely held opinion that eating more frequently is healthier than eating less frequent larger meals.
However, its adequacy for heterogeneous flows encountered more frequently is in debate yet.
The Department of Education clearly thinks that weighing the animal more frequently is more important than feeding it.
But accidental discovery might far more frequently be expressed in lyrics as form.
