UVC Lamp causes Atmega 328P to malfunction
While lots of folks are hypothesizing EMI, I would personally suspect that it is the photon flux from the lamp.  A UV germicidal lamp has pretty energetic photons with a pretty solid photon flux.  If you have some chip that isn't sufficiently opaque to the frequency of light emitted, you can get some weird effects.
For example: Xenon flash bulbs could reboot a Raspberry Pi 2 because they would generate carriers in the silicon and flood out the transistors in a power regulator chip.
https://www.alphr.com/raspberry-pi-2/1000375/why-a-camera-flash-will-reboot-your-raspberry-pi-2/
That lamp looks like an actual commercial product for aquariums, so I'm making a big assumption that it had to pass radiated emissions testing somewhere other than China.  Thus, I'm kind of skeptical that it is radiating enough to set off an EMI-caused reboot.
Good luck.