This code:
Private Sub Worksheet_Change(ByVal rngTarget As Range)
Debug.Print "Worksheet_Change" & "(" & rngTarget.Address & ")" &
Application.Ready
Application.EnableEvents = True
Application.Calculation = xlCalculationManual
rngTarget.Font.Bold = True
Debug.Print "rngTarget.Font.Bold " & rngTarget.Font.Bold
Application.Calculation = xlCalculationAutomatic
End Sub

Generates this output when editing a cell:
Worksheet_Change($BA$30)True
Got Here
rngTarget.Font.Bold True

But generates this output when picking from a data validation list:
Worksheet_Change($BA$30)True
Got Here
rngTarget.Font.Bold False


Ergo the assignment bold does not work and does not generate an error.