Founder of The Infraud Organization hacking group arrested in Moscow – source

This is Russia showing that being on good terms with it is useful for the US. It has the power to stop the ransomware problem if it wants to.

The implication being that if the US does too much about Ukraine (meaningful weaponry/sanctions) there won't be cooperation - but that if they do the bare minimum, well, their law enforcement might start mysteriously catching more people.

Nation states are big and relations between them are absurdly complex. You don't put all collaboration on hold every time a territorial scuffle is happening, especially when nothing explicit has actually happened yet. Just "tension".

I bet there is some non-public realpolitik happening. Maybe Putin is giving Biden news headlines in order to distract people and help him improving his falling rating in exchange to some future favors. Maybe not.

Besides geopolitics, simple answer could be that FSB protector who covered this guy finally realized that crypto goes down for long time, so it is better to take all collected funds now.

I'm speculating here but It's possible Putin changed his mind and now wants his share of their money which often counts to millions of dollars.