US 2 used today's WA 285 to WA 28 north until the mid 1970s.
I've read the
US Route 2 in Washington article in Washington before, but from what you said above, I just made the connection, though this sentence is still kind of vague, as it doesn't really say where the former alignment, but I can fill in the lines with what you said.
US 2 was routed north of Wenatchee onto the Olds Station Bridge, renamed in 1991 to honor Richard Odabashian, over the Columbia River in 1975,[67] while the former alignment was designated as SR 285 in 1977.
So my guess is that it was something like this for the former alignment of US Route 2 in Wenatchee/East Wenatchee:
https://goo.gl/maps/4iPansY392NVBeZv8I have my start point there as US Route 2 milepost 118 and my end point as US Route 2 milepost 128. With the way I routed it to get from point A to point B, Google Maps shows a distance of roughly 10 miles (10.2), so if I'm correct or roughly correct on what the former alignment of US Route 2 was in that area, that would explain the eight-mile gap as the routing I have fits the gap exactly. Although it makes me wonder if Washington State Route 28 didn't exist then or if US Route 2 was also (formerly) concurrent with Washington State Route 28 instead of just being concurrent with US Route 97 in that area at present time.
Whatever the case, with how important US Route 2 is, I'm rather surprised that they didn't adjust the mileposts in Washington to reflect the change from milepost 128 to the Washington-Idaho border in Newport, Washington, and vice-versa for US Route 2's mileposts on the westbound side between Newport and milepost 128 near the Washington State Route 28 junction in Wenatchee. For example, if they were to correct things, milepost 128 would actually be milepost 121 and so forth.