ik it used to happen somtimes when you alt tab in previous patch , but this patch it happen mid game and alot (at least for me)
There was something about the alt tab animation or something and how you could disable it and that might fix it. Even some Riot support guy said that, but I haven't tried it yet tho. I try to be more careful with alt tabs aka not instantly letting the keys go but hold them for couple seconds so the tabs can load. That has definitely helped me at least somewhat, but isnt permanent solution. This happens to me in Destiny 2 as well so isnt Valorant exclusive.