I noticed these about 4 years ago. The explanation is the phosphorus is failing. These are annoying. The deeper the UV is the more out of focus it is. I expect they are illegal under 2 different laws. Department of transportation laws governing lighting in general and health department laws laws since it has been proven UV kill rods and cones. I will be working to have these removed. Or they could simply buy phosphorus coated plastic or develop an aerosol can with phosphorus paint. My alter ego bicyclehazard has already forced over 300 departments of transportation to fix road hazards and violations of Title 23 Section 109m United States Code. I will give you advice on how to handle the government if you are interested.
There’s probably not much UV in it, if any. There are no health issues, but illumination is bad of course.
Assuming street lamps meet stringent gov regulations for the countries in which they are sold and installed, the poly carbonate lens used in street lamps block almost the entire relevant UV spectrum, meaning both UVA and UVB. Same with PC optics that have become extremely popular with emitter arrays.
Perhaps the question to ask is, “what are the materials used for the lens/optics in the effected lamps?”
Are you sure these are not the blue lights used in areas to dissuade drug addicts from shooting up?
Purple Rain?
A quick search shows it’s a national problem with faulty leds and they are getting replaced. Has your municipal just decided to keep them for 4 years? 
Call up the power company and/or city and let them know about them. Most of the utilities are getting compensated by their distributors for the faulty lights so they’re more than happy to replace them when they’re made aware of them.
/so much for LEDs being “environmentally friendly” when you’re having to throw out hundreds of thousands (or millions) of fixtures like this… ::sigh::
They are not like that here in Florida. They just installed LED streetlights in our neighborhood and are 4000k. You can read the 4000k sticker on the underside of the lamp and I also checked it with my Opple which agrees with the sticker.
This is definitely a problem in Florida. Sure, when these were installed they might be reading 4000K-5000K, BUT due to the Chinese factories using sub-standard parts (that’s never happened before, right?), the phosphors decay and you’re left with a very purple light. I’ve seen them in multiple counties and multiple electric companies in Florida. Like I posted above, the power companies will replace them quickly if you notify them of it.
I noticed this the last time I was in Lincoln, Nebraska. There were several street lights that were purple mixed among the normal yellow ones.
My immediate thought was that it seemed very peculiar, but then I thought that they were probably overpowered and the LEDs got burned out.
I accidentally overpowered some Nichia E17As, burned a black spot in the phosphor, and they turned purple.
Good to know, I’ll keep my eye on it as I have one in my front yard 
If it goes purple, I’ll take the opple outside again and see what the reading says before I notify FPL.