You may be thinking of a different film, as all the cast members listed on wikipedia[0] are stated to have died after 1945.

The trivia section of the German wiki page of the same film says there's a disputed rumour that the film was prolonged to help the young extras avoid conscription.

[0] https://en.wikipedia.org/wiki/Die_Feuerzangenbowle_(1944_fil...

Thank you for pointing that out! I think that's the rumor I heard...