I get the same issue - right click a panel > preferences, then after you close it, the icon remains in Plank. Clicking it toggles displaying the actual panel on/off. Clicking close does not remove it.
# apt list | grep -i plank | grep installed libplank-common/bionic,bionic,now 0.11.4-2 all [installed] libplank1/bionic,now 0.11.4-2 amd64 [installed] plank/bionic,now 0.11.4-2 amd64 [installed]
OS Release: Ubuntu MATE 18.04.1 Kernel: 4.15.0-43-generic
I get the same issue - right click a panel > preferences, then after you close it, the icon remains in Plank. Clicking it toggles displaying the actual panel on/off. Clicking close does not remove it.
# apt list | grep -i plank | grep installed common/ bionic, bionic, now 0.11.4-2 all [installed] bionic, now 0.11.4-2 amd64 [installed]
libplank-
libplank1/
plank/bionic,now 0.11.4-2 amd64 [installed]
OS Release: Ubuntu MATE 18.04.1
Kernel: 4.15.0-43-generic