Is the middle mouse button maybe also used somewhere else (e.g. for interaction)? This prevents the "copy block" from being executed unfortunately...
Nope, it's not assigned to anything else, I just checked it again before replying to make sure I didn't miss anything the last time. It was working fine before the seasons update, hasn't worked since. I tried it again earlier when I was playing and middle mouse button still isn't working for it, but a key bind on the keyboard works fine. And yes, I'm sure my middle mouse button is still functioning.