Everything works for me (Mojave with Safari 14.1).

I'm not saying the problem doesn't exist and/or that it's not Apple's fault, but, possibly, it's not strictly related to Safari.

DEBUGGING MINDSET: On MacOS, this Safari update was pushed along a MacOS update. (and maybe in iOS as well, along the 14.5 iOS?). How can you say the problem is within Safari? There's even a user complaining about a problem in Apple mail (a network timeout).

This problem could lie anywhere in the network stack, and maybe Safari is using a specific pattern that triggers the bug very often on some specific hardware. It could be a driver issue for what we know.

Now I'm trying to update my other machine (2015 Macbook Pro 15").