Problem:
Setting any customizatoin to mouse3,4 with any modifier has the side effect that randomly the default functionality of mouse3 mouse 4/sidebutton1 will misfire.
For example:
If I have middle clicking on menubar as mute toggle then regularly opening tab in background will misfire on first click then work on every click 100% of the time. Until some time later when I want to open some tabs again this will often happen in the same way: 1st click doesn't do anything but then it works fine.
Similarly for for button4 if I have cmd+button4 set to pause then sometimes when I'm going back a page in browsesr it will misfire the first click, nothing happens, then normal again with no misfire, but if some time passes and I want to go back again later, same story.
I've found out it's not a mouse issue since happens with all mice. Not a macos15 most likely since this happened to me for many months before I gave up and have been using Hammerspoon for these functions for a while.