The bourgeoisie very much took on the nationalistic project and the works of Finnish authors and kind of claimed them. The original print Linna wrote was never published as it was as the publisher demanded parts of it to be censored and the local bourge press turned the book into a moral panic as it went against their narrative. The original book name would be just "A War Novel" and today you can find it printed under the name Sotaromaani. In it he takes clear stances againts nationalism and militarist for example and these are things that have been purged from The Unknown Soldier. There is a study about it here in Finnish that might be translatable. Linna also personally held some anti-Soviet brainworms and some of these were also purged so it's complicated as well. But later the book and the movie both have been heavily utilized to advance nationalism, militarism and russophobia even though it was the Whites who used to hate this book.

It's a shame that most if not all of the actual leftist history here is these days underground or only in Finnish. But I do know that O.W. Kuusinen also wrote in English and these works can at least be read online.