MB5 binds remove themselves for some reason

So I have the first two item slots bound to my side mouse buttons, and the MB4 bind works fine, but for some reason, after a little while, the MB5 bind removes itself. So basically, nothing is bound to MB5 after it removed itself, and I have to first realize it in a moment when I need the bind and re-bind it again. Even though this is going on for about two weeks now, since I started binding stuff to the mouse buttons, I still could figure out a reason when and why it is unbinding. It feels like it happens at random times. Sometimes it works for several matches, and on other times it unbinds after two games.

Anyone has experience with that issue and maybe tips on how to fix it? This is getting extremely annoying since I want to put essential items in this slot and can't play the lottery with this bind every match.