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 commits since 1.2.0, and found that rPdd9f76525823: qt: Create all widgets in tab order fixes the problem.
If you could consider making a new point release from some point after that commit, it would be of great help to some distros.