Basically back when channelling was a thing I bound channelling to my fire button Mouse 1 this worked fine but I didn't really enjoy channelling and hadn't used it since before Melee 3.0 Phase 1. As a consequence I'm not sure if this was discovered as an issue when quick melee changed with phase 1 but now in phase 2 after a few hours of heavy attacks never occurring for me despite the Codex helpfully telling me all I needed to do was tap Mouse 1 with melee equipped. I discovered that heavy attacks wouldn't occur until I changed the key bindings. I suppose even with melee equipped it always wanted to swap to primaries, although I was able to play without my primary or secondary becoming equipped when I did tap Mouse 1 in melee mode. Hopefully any one else with this issue can see this and realise what's going wrong for them.