Why did my fruit tart cream/custard have spots in it?

I just made a fruit tart for the first time in my life. I searched for fruit tart recipes and I decided to use a 5-star recipe with the most ratings to try to minimize the possibility that anything would go wrong.

The recipe says the following for the cream/custard filling:

Ingredients: 2 cups whole milk, 1/2 vanilla bean, 6 egg yolks, 2/3 cup sugar, 1/4 cup cornstarch, 1 tbsp cold unsalted butter

1. In a medium saucepan, heat the milk and vanilla bean to a boil over medium heat. Immediately turn off the heat and set aside to infuse for 15 minutes.
2. In a bowl, whisk the egg yolks and sugar until light and fluffy.
3. Add the cornstarch and whisk vigorously until no lumps remain. Whisk in 1/4 cup of the hot milk mixture until incorporated.
4. Whisk in the remaining hot milk mixture, reserving the saucepan. Pour the mixture through a strainer back into the saucepan. Cook over medium-high heat, whisking constantly, until thickened and slowly boiling. Remove from the heat and stir in the butter.

The only thing I did differently from the recipe was to not include the 1/2 vanilla bean in Step 1. Instead, while stirring in the butter in Step 4, I also added 1/2 tbsp of vanilla extract. Besides that, I followed exactly what the recipe said to do.

So what happened is that during Step 3, while I was whisking the 1/4 cup of hot milk into the mixture, I started seeing some spots in the mixture. By the end of Step 4, there were a lot of spots in the mixture, as you can see:

Also, during Step 4, the cream/custard turned very thick very quickly, and I couldn't pour it out of the saucepan like the recipe mentioned. Instead I had to spoon it out into a bowl. So I guess I messed up the consistency, although the final result wasn't bad and seemed to taste fine.

So what caused all of those spots to appear in the cream/custard?