The only small arms cartridge plant that Ukraine had originally was in Luhansk, so it got captured even before 2022. It's only this year that they've got a new plant operational, but it produces both 5.45 and 5.56.

And Western supplies are mostly 5.56 for obvious reasons, although there are some exceptions - mostly countries that have switched fairly late and still have substantial stocks of 5.45, such as Bulgaria. But those are also limited in quantity.

So in practice it's not quite so simple, and Ukraine seems to be aiming for 5.56 as their primary cartridge long-term, specifically so that it's easier for Western countries to supply them with guns and ammo.