Wiktionary shows whereäs as a valid alternative spelling of the word whereas (see here).
It gives the following quotations to illustrate the usage:
1 Permanent International Association of Navigation Congresses,
Report of Proceedings — Milan (1905)
After a dry season, the influence of heavy rains becomes manifest more
quickly and more progressively in the
less wooded basin, whereäs the reverse
is found in the basin which has a
larger forest area.
2 Friedrich-Schiller-Universität Jena, Wissenschaftliche Zeitschrift
Notice that since the range of χ is limited by (5), whereäs that of x is
unlimited, the transformation (6) does
not cover the whole of the space-time
represented by (1).
3 Hiroyuki Sonoki et alii, Heart and Vessels — Nipradilol, a new
β-adrenergic blocker, reduces left
ventricular remodeling following
myocardial infarction in spontaneously
hypertensive rats (1997)
…after MI, whereäs propranolol promotes it.
That was the only dictionary where I found the word (with that spelling) though. And a Google search for whereäs returns only 209 pages.
Is that really a valid spelling of the word whereas? If yes, where did it come from?
English occasionally uses a trema (the two-dot diacritical mark over the “a” in whereäs) to signify diaeresis. Diaeresis means that a vowel combination that normally would form a digraph is instead pronounced as two separate vowels. In the case of “whereäs”, the additional trema gives us the familiar where-as pronunciation, rather than were-ease which would be suggested by the “ea” digraph.
Like so many aspects of English, usage of trema disappears as the original word becomes so commonly used that the diaeresis is understood. Thus whereas is now the typical spelling, rendering whereäs extremely uncommon, as you discovered.
The canonical example of an English word which includes a trema in the “standard” spelling is naïve.