Had this bug as well though in game it really didn't look like M1/M2 were doing damage so I thought they weren't doing anything. In the gameplay recording, it shows the proper character animation, with attacks, and damage output.
Eventually fixed itself again but Yamato in my view had the sword out and no animations for M1/M2 and what I thought was no damage output from M1/M2. There was a"[Space] Abort" button on the screen to the right of the crosshair that didn't go away with pressing space or zip lining, using ult or other abilities to figure out what I'm aborting. The UI skills were also greyed out but I could use them still, after the skill nothing was fixed, same for after death. It took ~2 minutes or so to fix itself again, perhaps after a combo of keys. Melee and skills being the main output for that time as I only think I saw M1 work a few times during it, but it might not have either, but the recording shows everything normal.
It looks like that dying during W, might do it.
Match ID: 194396 ~27 min death, 28 minutes to about 32mins when it was happening on my screen, where I melee and Q/E a lot vs M1/M2, then start to again