So let's say I'm a bank with a stack of 1B in high denomination central bank notes. I calculate the rate of return as zero minus the annual cost of securing those and the annual risk that they are stolen or destroyed. Inflation isn't a factor because the bond is in the same currency. Based on your explanation that rate of return will be lower than the -0.11% that I would get from a german 30 year bond today. And there's simply nothing I can exchange those central bank notes for that would do any better than the bond.
Like OP I've never understood negative yield debt and I'm trying really hard to.
There's also the fact that bonds can appreciate, so even if on the face of it you're taking an 11bp hit that might not be true in practice.