Having gotten grants and attended the "if you lie about using US money it is a felony" mandatory in person talk...
Well, it's not necessarily that they had foreign grants, though it is possible that would have made them ineligible for something. But not disclosing stuff like that is really bad.
I don't know why they didn't, the reasons might be innocuous in which case I doubt there would be harsh punishments.
Or maybe they got two grants from different governments paying for the same research (big no) or maybe they were simply given money by a foreign asset that now can blackmail them into disclosing secret research that the US funded.
The article doesn't really give that level of detail so I can only say it ranges from "mistake on your taxes" bad where the repercussions are mostly financial penalities and "stole money and gave nuclear missile designs to a foreign country because of the influence of their money".
If I were to bet: they either didn't keep good track of things, are the kind of people that simply don't care about the rules enough to know they broke them (even though they don't intend harm it's definitely their fault), or they would have been ineligible for funding if they disclosed the foreign source and were too greedy to take a pass on it.