Thank you but I am still unable to reproduce the issue.
I notice that instead of windows @a @b and @c in the Window list, you have AAA1 and AAA45, without the @ prefix. Which /window switches did you use to create AAA1 and AAA45?
Can you create a list of steps (not a video), like the steps in my previous post, that reproduce the issue for you?
I will then follow your list of steps.
1. Copy mirc.exe to a new, empty folder
2. Create an empty mirc.ini in this folder that contains:
[about]
portable=yes
3. Copy servers.ini to the folder
4. Run the mirc.exe
5. Enter nicknames in the Options/Connect dialog
6. Connect to a server with: /server gamesurge
7. Join channels with: //join #thisisatest1 | /join #thisisatest2
8. Open several @window with: //window -znkg1 @AAA2
9. Connect to another server with: /server -m freenode
10. Join channels with: //join #thisisatest1 | /join #thisisatest2
11. Open on the second network @window with: //window -znkg1 @AAA1
12. Select the @AAA2 window and close it
