With an auto, you should not be burned, even if it doesn't darken. It's not the visible obscurity that stops the UV. It's a shade 14 to UV in all states.

Do you wear safety glasses behind your hood? I did after I started do a lot of night welding and could see just how much light (incl UV) goes around the hood and bounces of my shirt and things behind me. Now, I'm religious about clear safety glasses behind my hood.

Yes, clear safety glasses also stop UV. I can walk around my shop when my guys are welding and not be bothered by stray UV. I'm a believer in safety glasses 100% of the time while working, doesn't matter on what.

Now, brightness IS brightness and can cause temporary black spots, sure, but if you got burning irritation, then something is wrong. I did a critical fit up the other day and forgot to turn on my BWE. I was holding everything in place and so I just tacked it without the hood darkening. It took about 6 seconds of arc, and after 15 seconds, my eyes lost the black spot and I was fine. I knew the hood, plus my clear safety glasses, were stopping the UV.

Are there any cracks at all in your lens assembly?