Today the groundhog.... did not see his shadow.
An uneducated worker/manager of the venue likely found germicidal lamps/LEDs that fit the sockets and figured it fits so it works. This happens all the time, and not just in Asia where clear labelling of dangerous light sources are an afterthought.
These lamps won't make you blind short term but they'll cause severe irritation and if not treated immediately, it will have an adverse effect.
Another reason could be the stage lights had improper bandpass filters (only allows certain wavelengths out), which allowed intense UV light to go through. Or maybe people at the venue removed the filters because it was making the lights dimmer.
Quick job is MSPaint.

Blue = proper
Green = cheap shit
While a filter like this would work for low power LEDs and cheap cameras, with an intense source, it won't do anything.
In fact, a piece of plexiglass might have been better than shoddy cheap filters in terms of UV filtering.
The fact that the attendee who blew the lid first claimed to have perfect vision leads me to believe that he obviously doesn't wear glasses, and attendees who didn't need glasses were the most affected. Decent glasses will have antireflection coating that blocks out lots of UV. Exposure to the skin is still bad though.
There were even utterly manic talks about UV-C potentially being able to kill airbourne covid-19, and this prompted warnings from professionals.
https://journals.sagepub.com/doi/full/10.1177/03015742211013761
An emergent health crisis, coronavirus disease 2019 (COVID-19), has demanded strict disinfection and sterilization protocols to limit the spread. One such approach is the use of ultravioletC (UVC) radiation for surface decontamination in hospital settings. UVC is the most powerful of the UV wavelengths and, therefore, the most damaging. The radiation is used for sterilization of workplaces and objects, and although there is no current evidence, perhaps it may eradicate airborne coronavirus. Humans should not be subjected to this form of irradiation. This form of radiation comes with a health warning, and humans should not be in the vicinity when UVC sterilization is occurring. This case report deals with the unfortunate incidence of an operator affected by UVC radiation while working on a patient in an orthodontic clinic. The article will serve as a timely warning to clinicians that awareness of the safety protocols should be observed. Besides, we present some recommendations for the use of UVC depuration of dental operatories.
Many years ago I posted about unregulated Chinese lasers with improper power ratings and shitty output from their cheap diodes. I'll emphasise again that this stuff is not a joke. If you want to play with weird lights, fine. Do 10 minutes of reading and get proper protection.