Switching to an SD+ stance then back to standing causes the currently selected FNIS Sexy Move animation to no longer apply. This happens even if your SD+ stance doesn't change (pressing the up arrow while already standing or attempting to change stances when not enslaved.)
If the 360 style walk/run animations pack from FNIS Sexy Move is installed it will also be unapplied.
In order to reapply the animations from FNIS Sexy Move you have to go back into the MCM menu and toggle animations every time it happens.