The New English Dictionary, later known as the Oxford English Dictionary, A–B (1889), terms this usage
improperly written together, when there is no unification of sense and while is purely a sb. [substantive = noun].
The first example given for this usage is William Caxton’s Faytes of Arms (1489):
It was doon but awhile agoon. It was done but a while ago.
“Improperly” written it may be, but in the 19th awhile — shall we call it a contraction? — commonly appears in a variety of genres.
When this insect is first hatched, it bears a complete resemblance to a common mite, but after awhile casts its skin, and undergoes a complete change. — The British Encyclopedia 6, 1809.
Here Rifault was well received by the inhabitants, who were Tupinambas ; after awhile he returned to Europe, leaving part - of his people under the command of Charles Sieur des Vaux. — Robert Southey, History of Brazil, Part 1, 1810, 392.
The whigs after awhile, cheerfully took the name ; but fastened on their adversaries in return an epithet altogether expressive of contempt ; ... — William Johnson, Sketches of the Life and Correspondence of Nathanael Greene, 1822.
And when at times a thicker darkness has seemed to gather before them, men have recoiled as from an impassable barrier, and for awhile that path has been closed. But only for awhile.— The Year-book of Facts in Science and the Arts for 1874, 1875, 107.
Heathenism was again for awhile restored after it had been rooted out. So much for the opposition of Satan against the success of the ... These heresies, which for awhile so much prevailed, after awhile dwindled away, and orthodoxy was again restored. — Jonathan Edwards, Dissertation of the End for which God Created the World, 1829.
A Google NGram suggests that this alternate was never more frequent than what the NED would consider “proper” English.