Hi Handoko,
your observation is correct (tested on Win64, Laz 1.8.0, FPC 3.0.4).
Documentation from link you included contains a demo Example91 which works as aspected in Win64, Laz 1.8.0, FPC 3.0.4. The only "uses" reference is there "SysUtils" but your example uses "Classes, SysUtils, Forms, Dialogs, StdCtrls" and may be even more as we cannot see the project source file. I guess that you are setting FormatSettings.DecimalSeparator global variable somewhere but not this one used by TextToFloat.
Summing up, TextToFloat has no bug.