I mean, honestly? That might be another explanation. Some combination of sanctions, effective counterintelligence, or "Ukraine is getting their weapons so what the hell are we paying you for" blocking the payments.
Russia's not running out of money any time soon (not on the scale of the laughably small amounts that it takes to corrupt people in the US government), but there are other reasons why people might not be getting their checks anymore. And Rand Paul is still going strong, where he's one of the people who's just motivated by some absolutely cockeyed internal compass that gravitates unerringly to the wrong answer on absolutely everything, so that would track, too.
Democrats vote for or against things all the time, whereas modern-day Republicans vote almost invariably in total lock-step (like literally 100% of them.) It's very weird. They have a whole little proto-Russia going on where everyone always agrees with the leader no matter what; it actually is sort of newsworthy at this point when some Republicans cross the aisle.