Once the war ends and both sides can start clarifying their troop classifications. There's always going to be uncertainty because an MIA could be dead, or it could be some guy who successfully deserted and started a new life for himself somewhere. But as both sides return captured troops, exchange bodies, and so on - everything will be made much more clear. And there will also be less political motivation to lie.

For Ukraine, war deaths would likely be a footnote compared to emigration when a new census is eventually completed (I don't mean to sound cavalier, but am trying to put things into perspective). An estimated 20% of their population has left since the start of the full scale invasion - ~10 million people - by now they've settled into new lives abroad (my 8 year old daughter's class here in Canada has 3 kids from Ukraine alone).

Ukraine is going to have some painful demographic issues to deal with when the dust settles (and I am cheering for them!).