One possibilty is that your uncle got trapped in the Festung Breslau which ended up being surrounded by the Russians. However Breslau was still fighting when all the surounding places had surrendered (I don't have the data here but if I remember correctly that was March 1945). It is east of the Oder in Silesia. If he managed to stay near the Oder, then it is possible that his unit withdrew to defend Berlin, but since you said he had orders to defend Hungary I would rather think that he was on his way when he was reasigned to defend Breslau, probably because the way to Hungary was cut from Silezia anyway and defending Breslau was a priority for Hitler.

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