It's not really about who started the war or who is invading who. It's 100% about pragmatism. You can't have an election when you're under threat of being bombed. You don't want large numbers of people gathering at a polling place, and people are displaced when there's a war and can't feasibly travel to where they live to vote. The Ukraine case has in addition to that parts of the country under occupation, so people living in those places obviously can't vote.

We should look at how it was with the UK in WWII. Elections were suspended, but after V-E day (Germany surrendered) they held an election. But this was before Japan surrendered. So they did hold an election before the war was over, because the UK was well outside of the range of Japan's bombers. Also note that Churchill lost that election, but he didn't dispute the results or anything like that.

And the US did have an election in 1944, again because there wasn't a danger from enemy bombers hitting theUS at that time.