Found as a follow-up to bug 1130227. For logs look at that bug.

The question is: why does it not propose swap to be in /dev/sdc if swap is
an optional volume (that can be disabled)? It's not logical.

After looking at the code indeed the idea in try_with_each_candidate_group in

https://github.com/yast/yast-storage-ng/blob/master/src/lib/y2storage/initial_guided_proposal.rb#L92

seems to be to reset the settings after each run. But disable_volume_property
in

https://github.com/yast/yast-storage-ng/blob/master/src/lib/y2storage/proposal/settings_generator/ng.rb#L101

modifies initial_settings and reset_settings

https://github.com/yast/yast-storage-ng/blob/master/src/lib/y2storage/initial_guided_proposal.rb#L172

has not the desired effect.