"What happens to weapons sent to Ukraine? The US doesn't know."

Some weapons might be captured by Russian soldiers. Others might end up in the black market, or in terrorists' hands... A few RPGs were found by the Russians.
Apparently this has been going on for a while, even before the current war...
https://www.militarytimes.com/off-duty/gearscout/2018/05/22/ukrainian-troops-spotted-with-american-made-but-still-russian-styled-weapons/