When opening the application the column width for the first tab is automatically optimized and is ok.When pressing new connection, the column width for Remote Desktop, Last Connected, and Created columns are too narrow. The same thing happens if you close your only connection (without closing the application). When pressing the Disconnect button the column width optimization for the appearing table is gone. This affects both fullscreen and windowed connections mode.
As of KRDC version 18.04.3 I don't have the issue any more. I guess it was a Gentoo specific bug.