In my case, I have USO and UCO...these are oil price ETFs, dealing with futures and contracts. The document says they will send a K3 later, and a limited number may need it. I think you are right, since I am in the US and US resident for tax purposes, it should not be required for me.
This is what the document they provided says:
"Beginning in tax year 2021, items of international tax relevance are now provided by the Partnership on Schedule K-3. A limited number of
unitholders (primarily foreign unitholders as well as certain corporate and/or partnership unitholders) may need the detailed information disclosed
on Schedule K-3 for their specific reporting requirements. The IRS has recently provided additional Schedule K-3 guidance, and we will continue to
monitor any future changes impacting our partners. Accordingly, the Schedule K-3 is not included in the enclosed tax package and will be
available online at www.taxpackagesupport.com/proshares by August 31, 2022.
