Seems to be a bug. A workaround is to handle SystemEvents.UserPreferenceChanged for e.Category=VisualStyle and change the progressbar style to something and back again.
Imports System
Imports Microsoft.Win32
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
AddHandler SystemEvents.UserPreferenceChanged, AddressOf SystemEvents_UserPreferenceChanged
End Sub
Private Sub SystemEvents_UserPreferenceChanged(ByVal sender As Object, ByVal e As UserPreferenceChangedEventArgs)
If e.Category = UserPreferenceCategory.VisualStyle Then
Dim CurrentStyle As ProgressBarStyle = ToolStripProgressBar1.Style
For Each Style As ProgressBarStyle In [Enum].GetValues(GetType(ProgressBarStyle))
If Not Style.Equals(CurrentStyle) Then
ToolStripProgressBar1.Style = Style
Exit For
End If
Next
ToolStripProgressBar1.Style = CurrentStyle
End If
End Sub
End Class