Not a bug. You are sending a signal that has too low samplerate to ever be 1600 horizontal active.
432*3 = 1296
The output is not valid, you get a negative frontporch.
To get a valid mode, sync and h backporch need to be less than 112 together (so that h frontporch can be at least 1).
You are trying to use a mode which is not really intended for such low samplerates, nor the aspect ratio of that video. If the signal is multiplied 5x vertically, but only 3x horizontally, it will be way way too skinny (or not tall enough, in the case with TATE game content, which is exactly what you describe already).