With pinentry-qt 1.1.0, running
echo getpin | pinentry-qt --display "Test"
would display a *floating* dialog box under the Sway WM. This is the desired behavior. As of 1.2.0, it instead displays a "full-size"/tiled window.
I bisected through upstream's later commits, and found that `dd9f765258230cad6704afb4fab6c3deb4a8de56` fixes the problem.
If you could consider making a new point release from some point after that commmit, it would be of great help to some distros.
See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1013318