That's quite a plant! But I can't help you as I don't know what it is and have never even see one. It might help if we had a general idea where you are. When you say "full sun", elevation makes a difference (we have real "full sun" in Albuquerque, so full it would scare plant lovers with experience only of sea-level gardening). Also, are you in a humid or a dry area, average temps, that kind of thing.

Edit to add: I think I've read the name Puya before, maybe it's South American plant that likes dry? "Alpestris" almost sounds like "Alpine", but then the "pest" part of the name... I hope it's not a weed in it's home territory! :)

Too much direct light, from what I've read. Dying tips on Bromeliads usually means too much light. It also matches up with what you said, because you moved them out of shade.

They're still technically seedlings, and even though they're from a hardy plant, they're not as hardy as the adults.

Either they will continue to suffer until you move them back to the shade, or they will probably slowly loose all their current leaves, but grow new ones adapted to the full sun.

Although only one is being hit the hardest, you can see it starting on the one in the upper left corner plant too.

I would recommend hardening them off. You can find advice time details online, it's pretty common. Basically you move them in and out of the direct sunlight, gradually increasing the length of time they spend in full sun before finally leaving them there.

Puyas might be okay without it (I have no personal experience), but if you do leave them out there, they probably will end up with the older foliage scorched or dead. Some species take it better than others. Adeniums, for example, prefer to be gradually exposed, but you can just stick them out in full sun, and all their leaves will scorch, but the new ones will come in adapted to the full sun. Aloe Vera will straight up die if you try to immediately put it in full sun without giving it time to adjust in my experience.