gear box issues

I have a zxr750h1 when changing down from third to second it drops from second to neutral. This does not happen on low revs only when trying to use a lot of engine braking ,there is no problem on up shift any ideas on my problem would be good .My thoughts are that the shift brum is over rotating

I reckon it could be a worn selector fork more than a drum issue. If the fork is worn, it may be sliding the cog halfway and engaging angel gear. It could also be worn dogs making it slip out. Either way, it's an engine upside down job and have a look.