Another possibility is that the russians have taken radio-active material out of a war zone, which in this case would be the responsible thing to do. Or could be a clueless solider who stole it. God knows.
Anyone sane can logically calculate that Russian does not have any needs to loot dirty bomb materials. They have plenty enough themselves or capability to manufacture any amount more they need.
Do dirty bombs have a fingerprint that could be traced to a Ukraine source?
Yeah, absolutely. The radioactive signature is highly traceable. IIRC, they were able to pinpoint the source of the polonium that Russia used to murder Alexander Litvinenko in the UK.
Unless they need the world to see that the radioactive stuff came from Ukraine.
Like if the world wasn't aware Chernobyl is under Russian control?
If Russia is responsible for the missing materials it could easily be part of a propaganda campaign. This specific scenario has been discussed a lot in the last few weeks