I've been looking at French family trees at Geneanet, being "presque courant", and I've found clusters of Soehnlens around Haut-Rhin in southern Alsace. One town or commune that comes up often is Thann.
Turns out that a church in Thann is dedicated to St. Theobald (Thiébaut), which might be a valuable clue.
Could it be that infants are commonly named after a locally venerated saint? Would that be a middle name or christening name or whatever you call?