I think I found the cause of the problem.
previously i used: tft.fillRect, tft.fillScreen (TFT_BLACK), tft.setTextColor (TFT_WHITE)
But it seems more correct to use tft.setTextColor (TFT_WHITE, TFT_BLACK), that is, not a transparent background and then tft.fillRect, tft.fillScreen (TFT_BLACK) is not needed.