Aphantasia is a condition I have never heard about. Your experience and the description on Wikipedia lets me think I was born like this as well.

I had many arguments with my wife about how I did not have visual dreams and did not see anything when I close my eyes. When she says she sees stuff when she closes her eyes, I always assumed it was some sort of metaphor and it was usual for human beings not to see anything when their eyes are closed.

I think when I "visualize" something, not limited to math and code, I do not see anything per se. Mostly I think my thoughts are directed graphs whose nodes are concepts I have in memory.

May I ask how did you get a diagnosis? And did you learn anything of use thanks to this diagnosis?