Function Keys Keybind

So, I was trying to rebind my F5 key to my mouse5 button (I'm using a razer mamba TE). The problem is, when I hit the button, I have to hit a keyboard key before it actually changes. I thought it might be a problem with the mouse button in question, but the same happens with every other mouse key. However, if I change it to a keyboard key, it works just fine. The same happens for every other function key. I've tested this on both 1.8 and 1.16. I had worked on 1.16 in the past, but suddenly stopped working when I moved to 1.8 and back to 1.16 (I did re-bind the key). Oddly enough, sometimes it does seem to work perfectly fine on 1.16, but other times, it does the weird glitchy thing on 1.16 as well. I'm just really confused as to whats happening and if I can fix it. I use the f5 button quite often but its just a LOT easier if I have it set as a mouse button.