Not in wartime, but in recent years, the British Museum, a prime holder of foreign antiques, reported they lost thousands of items. Some were found or recovered, but some were confirmed as stolen.

https://www.bbc.co.uk/news/articles/crgy4w221z5o

This list is terribly incomplete. It's missing all of the artifacts stolen by the Soviet Army during WWII, and still not accounted for.

The USSR systematically stole from all the areas they occupied. They stole the indexes and inventories, so it's not known what was taken.

Entire libraries and museums just vanished.

The Soviets implied they were destroyed in the war, but there is strong evidence the Soviet system of looting preserved much. In the early 1990s, some treasures started to leak out.

Wikipeia is good at some things, and terrible at others. Compiling a list of "missing treasures" is far beyond it's competency.

That is a very short list of particularly notable lost items. European museums had hundreds of millions of items in their collections, and many of them were lost. The number of documented pieces of art and artifacts lost in WW2 is in millions. The actual number is likely much higher, as the documents were often also lost. Nazis looted as a matter of policy. Soviets practiced wholesale destruction. Even Americans did widespread looting, often taking items that had first been stolen by Nazis.

For a more concrete example: The Colonial Collections Committee report behind the article discusses a total of 390 items. 10 of them were known to be lost before the 1990s. Further 17 were not found in an inventory at that time. One additional item was determined to be a loss that had been reported earlier. Two further items had been reportedly transferred somewhere else, but their current status is unknown.