Mandos declares in the Prophecy of the North (aka the Doom of Mandos) that the House of Fëanor shall never recover the treasures they have sworn to pursue. Nevertheless, following the War of Wrath, Maedhros and Maglor manage to steal the two remaining Silmarils, practically from under the very nose of Eönwë himself.
It is true that, because of their evil deeds, they are unable to withstand the power of the Silmarils, a fact which drives them to deliver the stones to their ultimate resting places, but at least for that moment, they had regained what they had sought, in apparent conflict with the Doom of Mandos.
I recognize that it could be argued that the very words of Mandos ring true, that the treasures are ultimately “snatched away” through their Oath, and the deeds done under it, though in this final resolution, the Stones meet their fates by the hands of Maedhros and Maglor themselves. If this constitutes being snatched away, then it is indeed a highly metaphorical interpretation of the phrase.
The ultimate fates of the Silmarils, and how they arrive at those fates, are something that Tolkien himself considered very carefully, and rewrote multiple times. As such, this can hardly be considered a continuity error, knowing how carefully this sequence of events was assembled. I would simply argue that it’s a result of misinterpretation of the words themselves, which perhaps lost something of their original meaning when translated from Quenya to Westron to English.