Yes, the issue is still there.
How to reproduce it:
You need to start adding panels to your laptop screen. After all four sides are filled, the next panel will be added to the external monitor. Then you just remove the unnecessary panels from the primary (laptop) monitor, leaving you with what you need on the primary and secondary display.
After you remove the secondary display (external monitor), the panels from there will be added to the primary (laptop) screen.
This is not good, or definitely should not be this way.
After reconnecting the external monitor, at least now the panels are put back to it again, so they don't remain on the primary (laptop) monitor, but transferred back to the secondary (external) display.
That's what changed since.
The original issue is still there though, which is transferring the panels from the secondary screen to the primary screen upon disconnection.
Yes, the issue is still there.
How to reproduce it:
You need to start adding panels to your laptop screen. After all four sides are filled, the next panel will be added to the external monitor. Then you just remove the unnecessary panels from the primary (laptop) monitor, leaving you with what you need on the primary and secondary display.
After you remove the secondary display (external monitor), the panels from there will be added to the primary (laptop) screen.
This is not good, or definitely should not be this way.
After reconnecting the external monitor, at least now the panels are put back to it again, so they don't remain on the primary (laptop) monitor, but transferred back to the secondary (external) display.
That's what changed since.
The original issue is still there though, which is transferring the panels from the secondary screen to the primary screen upon disconnection.
Ubuntu 20.04.2 LTS x86_64 (Ubuntu MATE)
5.4.0-80-generic
MATE 1.24.0
Metacity (Marco)