Based on the below article, the 6.5T is the amount maturing for 1H2025, it is not a standalone Jun month figure.
https://finbold.com/9-trillion-of-us-debt-will-mature-in-2025-should-investors-be-worried/
The entire refinancing amount for 2025 will be 9.2T, plus new issuance, maybe another 2T more. Last year's refinancing figure was also around the 9T range.
US treasuries are held by a lot of people, a sell-down does not mean sovereign investors are the only ones selling. Looking at the major foreign holder listing published by the Treasury, that table is likely to be referring to holders by country of origin rather than central banks. If we look at the UK figure of 740b vs the published official reserves of 100b++, it will not tally. The same goes for the Cayman Islands holding 400b worth of treasuries. Both the UK and Cayman are popular jurisdictions for fund management.
https://ticdata.treasury.gov/resource-center/data-chart-center/tic/Documents/slt_table5.html