It revolves around driver, but a fact, that it is a regression, speaks that the driver is not the cause.
Should be a change in the engine or any of its third party dependencies.
NVIDIA forums have a 40 page long thread of complaints about stuttering on driver, that follows 385.69.
A more or loss overall consensus was that it is caused by polling GPU power via nvapi.
But no word on memory leak. Seems unrelated. Besides, it would be pretty strange, if such major was ignored by Nvidia, if it existed.
Fornite memory leak regression complaints(like this one) are all over the place though.
Number of UE4 devs or end-users, suffering from the, is unclear. With current theory, it should be everyone with nvidia gpu, running post 385.69 driver.
20 votes on the tracker UE-58676 and UE-57557 and just a few people posting here and mere fact that Epic could not miss such major regression during QA(or could they?), kinda hints that there is something else in the formula.