Mac OS X 10.14.5
Since upgrade to Alfred 4 ejecting of encrypted sparseimages is not permanent.
Steps are reproducable and consistent for all sparseimages (I created new, clean sparseimages and confirmed).
1.) Mount an encrypted sparseimage
2.) Enter password in dialog, do NOT select "Remember password in keychain"
3.) sparseimage will mount as expected
4.) Eject the sparseimage using the "Eject" command in Alfred
5.) Mount the sparseimage again
6.) The sparseimage will mount WITHOUT ASKING FOR A PASSWORD.
If the sparseimage is eject any other way (for example clicking the "eject" icon in the finder) then it will, as expected, ask for the password before mounting.