I can confirm this bug. Note that it is not just a problem with custom .gtkrc files, it occurs with regular themes too. For example, if you install the Murrina-LiNsta theme you'll see the problem.
From the upstream report, a workaround is to put the following in your .gtkrc:
widget "*fast-user-switch-applet*" style "panel"
where "panel" is whatever style is being appled to PanelApplet widgets.
I can confirm this bug. Note that it is not just a problem with custom .gtkrc files, it occurs with regular themes too. For example, if you install the Murrina-LiNsta theme you'll see the problem.
From the upstream report, a workaround is to put the following in your .gtkrc:
widget "*fast- user-switch- applet* " style "panel"
where "panel" is whatever style is being appled to PanelApplet widgets.
I can confirm that that works.