I have a 3 monitor setup with the following plugin config:

plugin {
    split-monitor-workspaces {
        count = 10
        keep_focused = 0
        enable_notifications = 0
        enable_persistent_workspaces = 0
    }
}

windowrule = workspace 2,^(thunderbird)$

Workspaces 1-10 are assigned to my right monitor, 11-20 to my left one and 21-30 to my center one.
If i am currently on my center monitor and open thunderbird with rofi, it does get sent to a workspace with the name "2", but the workspace will be on my center monitor instead of on my right one.

Expected behavior would be that the workspace number 2 will also spawn on the right monitor since it was assigned to that screen by the plugin.

If i enable persistent workspaces everything works as expected.

I can provide more info if needed / test other things.