Sometimes if you swing or shield right after a Winmillion charge it may not shoot out it's projectile, it can, but not always.
As while the charge is going off you can hold down to charge again which causes another combo swing, or shield if you're gonna be in danger after charging.
It'd be nice to have it always shoot out a projectile or shoot nothing at all.
I've been able to get ASI Max or Very High with my gear, with my Winmillion and haven't tested much without it so maybe that might also contribute to this.
I don't know if it's related to server lag or my lag or fps or my ASI, but this is something that's always bothered me and the Winmillion is one of the first 4* weapons I've made and always used.