According to an article in Financial Times, the shipment was seized in Bangkok, though 3M currently denies that any such order existed:
Andreas Geisel, Berlin’s interior minister, confirmed that the consignment had been “confiscated” in Bangkok and never reached Berlin.
“We consider that an act of modern piracy,” he said. “You don’t treat your transatlantic partners like that.”
3M disputed the account, saying it has “no record of any order of respirators from China for the Berlin police”. German authorities were not available on Friday evening to explain the discrepancy. ...
According to Tagesspiegel the Berlin police had ordered the masks from 3M. They were produced in a 3M factory in China and were due to be reloaded at Bangkok airport and flown to Germany. Instead the delivery went straight to the US.
US swoop sees 3M masks allegedly diverted from Berlin - Financial Times
Apparently, this isn't the only case. That same article mentions complains from France, while even US states have had their medical supplies seized by the Feds:
Warren also pointed to at least two instances in which Massachusetts lost orders for ventilators and hundreds of respirators after the federal government stepped in — a situation Marylou Sudders, the state’s health and human service secretary, described as: "They take what we order.”
Sudders said the Trump administration also impounded a third order of 3 million masks the state had negotiated to buy earlier in March. ...
Warren asked Gaynor to explain why the federal government seized the two orders from Massachusetts, and to which state the supplies ultimately went, among a series of other questions in the letter.
Warren to feds: Why did you take Massachusetts’ medical supplies? - Boston Globe
Another source on the same story:
Gov. Charlie Baker said a complicated international plan to purchase over 1 million N95 masks for Massachusetts health care workers and first responders was hatched after a previous order was confiscated in the Port of New York.
"Around the time that we had our 3 million masks that we had ordered through BJs confiscated in the port of New York, at that point it became pretty clear to us that using what I would describe as sort of a 'traditional approach to this' wasn't going to work," Baker said Thursday.
The governor did not specify the agency responsible for confiscating the masks. ...
The coalition arranged a purchase of the masks in China, which were flown back to Massachusetts Thursday aboard a New England Patriots private plane.
The supplies were then transported by the National Guard to a stockpile in Marlborough.
3 million masks ordered by Massachusetts were confiscated in Port of New York, leading to creative alternative