[Ger-Poland-Volhynia] GERMAN CITIZENSHIP MAINTAINED?

[Ingrid Abram]

My mother-in-law said her father, Karl Heinrich Heinemann,
born 1861 in Tuczin, Wolhynia, had citizenship from Leipzig
which allowed his children to go to Germany after their
Siberian excile.

Have not been able to document this.  Is this true for others
from Wolhynia?  Or is it possible he served in the German Army
before his marriage?

Ingrid Abram