Describe the bug
The version of KeePassXC in nixpkgs is 2.3.4, released 2018-08-24. The last release is 2.4.3. There are many usability improvements in the 2.4 series.
It's already in unstable master: https://github.com/NixOS/nixpkgs/pull/66572
Indeed, it's in unstable and will reach 19.09.