On my system: $ rmadison linux | grep bionic-proposed linux | 4.15.0-48.51 | bionic-proposed | source
$ apt show linux-generic Package: linux-generic Version: 4.15.0.48.50
So I think -proposed is not correctly enabled in /etc/apt/source.list.d. This can be a bug in the updater GUI.
On my system:
$ rmadison linux | grep bionic-proposed
linux | 4.15.0-48.51 | bionic-proposed | source
$ apt show linux-generic
Package: linux-generic
Version: 4.15.0.48.50
So I think -proposed is not correctly enabled in /etc/apt/ source. list.d. This can be a bug in the updater GUI.