SUMMARY
kcm-settings for a "wired shared"-connection does not allow to specify "ipv4.addresses". If connection has that setting (which works fine) it breaks the functionality (removes the setting) upon saving (clicking "apply").

When using a connection of type "wired shared" the GUI does not allow to enter something under "ipv4.addresses". It automatically selects a dhcp-range and gets itself an IP-address.
If you configured an address using nmcli like "192.168.2.11/24" and editing that connection this address shows. And it works correctly in NetworkManager (you get assigned that fixed address, the prefix is used as DHCP-range).
But if you try to save the connection in the kcm-settings (probably since it is grayed out) it breaks the configuration you entered using nmcli. You can however use nmcli again to re-add that additional configuration.


STEPS TO REPRODUCE
1. Add a connection of type "wired shared"
2. Try to configure a given IP-address and dhcp-range for that connection. (you can't)
3. So use nmcli to configiure that setting. (works)
4: Edit the connection, simply save it again.

OBSERVED RESULT
1. kcm does not allow to specify a setting for ipv4.addresses
2. Editing a connection which has that setting breaks things (removes the setting) upon saving.

EXPECTED RESULT
1. Allow specifiying that setting (perfectly valid, works fine with NetworkManager).
2. Don't break additional settings upon saving.

SOFTWARE/OS VERSIONS
Fedora 42, kf6-networkmanager-qt-6.18.0-1.fc42.x86_64