There are several official and unofficial sources that say that the public domain status of works of the US Federal Government only applies within the US.
"Government Works" on USA.GOV
According to "Copyright Exceptions for U.S. Government Works":
U.S. copyright laws may not protect U.S. government works outside the country. But the work may be protected under the copyright laws of other jurisdictions when used in these jurisdictions. The U.S. government may assert copyright outside of the United States for U.S. government works.
Government Copyright FAQ on cendi.gov
According to Frequently Asked Questions about Copyright: Issues Affecting the U.S. Government (CENDI/2008-1) (Revised December 2017) FAQ Item 3.1.7:
3.1.7 Does the Government have copyright protection in U.S. Government works in other
Yes, the copyright exclusion for works of the U.S. Government is not intended to have any impact on protection of these works abroad (S. REP. NO. 473, 94th Cong., 2d Sess. 56 (1976)).
Therefore, the U.S. Government may obtain protection in other countries depending on the treatment of government works by the national copyright law of the particular country. Copyright is sometimes asserted by U.S. Government agencies outside the United States.
US House report on the 1976 Copyright Act
According to that part of the "House report no. 94–1476" included as "Historical and Revision Notes" to the test of 17 USC 105 in the version of "TITLE 17—COPYRIGHTS" on Govinfo
The prohibition on copyright protection for United States Government works is not intended to have any effect on protection of these works abroad. Works of the governments of most other countries are copyrighted. There are no valid policy reasons for denying such protection to United States Government works in foreign countries, or for precluding the Government from making licenses for the use of its works abroad.
Public Domain Sherpa
According to "Public Domain Sherpa":
US government agencies may claim copyright abroad
While US government works generally are in the public domain in the US, they may be protected by copyright abroad. The feds may claim copyright protection for US government works in other countries depending on how those countries treat their own government works. So just be aware that US government agencies sometimes claim copyright in their works outside the US.
Protecting Government Works
According to the section "Copyright Protection Abroad For Government Works" of "Protecting Government Works: The Copyright Issue" by Paul C. Manz, Michael J. Zelenka, Raymond S. Wittig, and Sally A. Smit in
... If another signatory to the Berne Convention, however, allows copyright protection of its government works (i.e., Crown Copyright of the United Kingdom31 or Canadian Copyright Act32), it is clear that that country’s courts must afford protection to similar works of the U.S. Government, despite the fact that the work could not be protected in the United States.
Congressional legislative history supports this position. [The document goes on to quote the same passage of the house report notes to 17 USC 105 quoted above.]
... For a work to be eligible in a foreign country for copyright protection, it must qualify under the particular laws and requirements specific to that country (i.e., in the United Kingdom copyright requirements are provided in the Copyright, Designs and Patents Act 1988). Other countries and their laws may differ from the requirements for copyright protection as set forth in the United States Copyright
Article 5 Paragraph 2 of the Berne Copyright Convention specifies that (emphasis added):
... such enjoyment and such exercise shall be independent of the existence of protection in the country of origin of the work. Consequently, apart from the provisions of this Convention, the extent of protection, as well as the means of redress afforded to the author to protect his rights, shall be governed exclusively by the laws of the country where protection is claimed.