it is common to have around 14.6 volts when running.
like mentioned a few posts back, electrical interference from the alt may be causing static that the flasher is sensitive to. you may need a different flasher that isn't so sensitive. you may also have just received a dud flasher.
some interesting reading
https://www.youtube.com/watch?v=9PyC_tFHKJE
https://www.motortrend.com/news/ligh...-flasher-leds/