I don't think it's that - or rather said, I don't think that's the primary or even the biggest reason.
If that were actually the case, you would expect to see a gender disparity in the 20-25 age range because women aren't conscripted into the army.
There's little to no gender disparity there in that graph.
What I think the cause is for that enormous dip is precisely the period when those babies were conceived.
20-30 year olds were conceived in the 1990s-2000s, a period of time which was quite difficult for Ukraine, subsidies given to mothers were low, often not even paid in full, applying for benefits was so complicated it barely was worth the effort of doing so.
In just a couple of years after the collapse of USSR, life expectancy dropped dramatically in Ukraine, so that should give you an idea of how that period of time was for the people.
Reality is that when the economy is doing shit, people won't have babies.
Check this fertility drop post '89: https://i.imgur.com/dvADJ9i.png
And why don't you think that the low birth rates of the 90s-2000 is a valid explanation?