If you do code in any capacity not being able to understand long division is a huge red flag. In terms of difficulty it's somewhere between regex and being able to write a recursive function.

I didn't understand why it worked (a comment here pointed out that it's just division-by-repeated-subtraction which does make sense now). But I was still able to perform it, during the time when it was relevant to my life.

What's a red flag is being frustrated by long division and at the same time not being curious or resourceful enough to resolve your frustration. You blame your lack of understanding on other people.

I can understand if most people shared your frustration, but not a PhD holder. Your degree basically says that you're capable of doing long, intensive research on arcane topics.

What am I missing here?

From the other comments that user's posted I can kind of understand their sentiment, they've tried to form an understanding from others and then found the same roadblocks repeatedly.

I have impaired executive functioning and this is basically my experience in tying shoelaces, I struggled to accommodate the mental model of the process myself so I sought out others so manage it to try and learn from them.

What instead happened was exasperation on their part and frustration on mine, so after repeating that a few times a year until I was 22 I just stopped and bought shoes without laces.

When you go at it for long enough without succeeding it can make the experimentation not very fun or interesting.

I don't think I'd call it a red flag for a PhD to have fatigued themselves out of interest in a problem, it's something that happens to everyone at some point.

Sometimes even if you drag your brain kicking and screaming through a mathematical topic, it just doesn’t give a shit. I felt the same thing about polynomial long division back in high school — it was just a bunch of gobbledeygook — so I’d forget how to do it, and then divide polynomials by ad-hoc trial multiplication/subtraction, until I figured out the most convenient way to do that was to first cancel away the leading term. And today, I’m uncomfortable with multivariable calculus. I’ve made a few abortive attempts to acclimate to it, and I’m sure I could force myself to get there if I needed to, but right now that’s just not important.