Dimitri lived in Denmark and was always styled by the Danish court as prince so maybe the Russian authorities simply followed the example of the country he lived in (possibly the title was in his Danish passport).
In that case the title would have been a Danish one as Imperial Russia never had the title prince Romanoff.


--Previous Message--
: Dimitri Romanoff, the younger son of HH Prince
: Roman of Russia and Countess Prascovia
: Cheremeteva, died 31 Dec 2016 in a hospital
: in Denmark. He was 90. He is survived by his
: widow, Feodora (née Dorrit Reventlow). He
: had no children.
:
: As President of the Romanoff Family
: Association he was also the head of that
: association's charitable foundation which
: raised money for several causes to benefit
: the people of Russia. The bequests were
: often medical equipment and supplies.
:
: Dimitri's position in the family was
: disputed. According to the generally
: acknowledged Head of the House, Grand
: Duchess Maria, he was not a member of the
: Imperial House, the same opinion she
: currently has of every other Romanoff,
: except her son. In her view, Dimitri was
: simply Mr. Dimitri Romanoff.
:
: Dimitri's father, Prince Roman Petrovich,
: was of the opinion in 1921 (when he married
: Prascovia) that there was no longer an
: Empire, and there was no longer an Emperor,
: so there was no one that he had ask
: permission from to marry. As a result, he
: never asked for, nor did he receive, any
: title for his wife or children. By Roman's
: thinking his children would be HH Prince
: Nicolas (d.2014) and HSH Prince Dimitri of
: Russia.
:
: The current Russian government, which
: recently awarded him the Order of Alexander
: Nevsky for his charitable work, refers to
: him as "Prince Dimitri Romanoff."
: