This issue should be reported to wxWidgets. But I think GetStyledText is rarely needed. 
From your previous post, I guess you are trying to get the style number of the text.
FYI
The style number that GetStyledText returns is the number that the Lexer automatically assigns to the token. For example, if SetLexer(stc.STC_LEX_PYTHON) is used, one of the following 16 styles are assigned to each token:
wx.stc.STC_P_DEFAULT 0
wx.stc.STC_P_COMMENTLINE 1
wx.stc.STC_P_NUMBER 2
wx.stc.STC_P_STRING 3
wx.stc.STC_P_CHARACTER 4
wx.stc.STC_P_WORD 5
wx.stc.STC_P_TRIPLE 6
wx.stc.STC_P_TRIPLEDOUBLE 7
wx.stc.STC_P_CLASSNAME 8
wx.stc.STC_P_DEFNAME 9
wx.stc.STC_P_OPERATOR 10
wx.stc.STC_P_IDENTIFIER 11
wx.stc.STC_P_COMMENTBLOCK 12
wx.stc.STC_P_STRINGEOL 13
wx.stc.STC_P_WORD2 14
wx.stc.STC_P_DECORATOR 15
If you want to get the indicator number such as stc.STC_INDIC_SQUIGGLE, the following methods should be used:
IndicatorValueAt(indicator, pos) -> int
IndicatorStart(indicator, pos) -> int
IndicatorEnd(indicator, pos) -> int
IndicatorAllOnFor(pos) -> int