This bug is similar to #9995, #12670 and #10725. With a two-monitor setup, each monitor having a panel with window buttons showing windows from that monitor only, the right-hand monitor sometimes shows the windows from the left-hand (primary) monitor. The workaround is to enable then disable the "show windows from all monitors" setting on the affected window buttons. The difference between this bug and the ones mentioned is that this is not restricted to startup. It happens at seemingly random times.
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