I use a portable version of Krita on my Windows 7 machine.
The lasso tool ignores shift/alt keys and always selects a new region.
It is possible to add/substract if modes are selected in Tool Options.
Rectangular, elliptical and polygonal tools work just fine, but not the lasso for some reason.
STEPS TO REPRODUCE
1. Select a region using a lasso tool.
2. Hold shift
3. Try to add a region to the selection.
A new region is selected with the old one being discarded.
A new region is added to the old one.
This got fixed today
*** This bug has been marked as a duplicate of bug 410532 ***
Solved as of https://invent.kde.org/kde/krita/commit/bd06296f66a4620099c59bdee5a85d07fbb61d39
Thanks for reporting, :3