I configured my mouse wheel to be reversed in systemsettings.
Since the upgrade from 5.12.x to 5.13.x (at least) this setting is not applied any more after login - the mouse wheel now behaves in the traditional way.
I have to open systemsettings, unselect the checkbox "reverse mouse wheel", select the checkbox "reverse mouse wheel" again and press "apply" in order to fix this.
That's rather annoying, it worked just fine for many releases.
Please reassign this bug to the right product / component if plasmashell is wrong. I wasn't sure if I should assign it to "systemsettings", but as systemsettings seems to work fine and plasmashell just does not pick those settings up after login, I assigned it to this product.
*** This bug has been marked as a duplicate of bug 395401 ***