So, I've run into something that I'm like 90% certain is a glitch because I refuse to believe I'm just lucky enough for it to happen.
I've started two files, one on mac version 19 and one on pc version 19.03 and in both I encountered a wild shiny pokemon (bellsprout on mac and hoothoot on pc) and then, staying in the area and continuing to grind in the grass, I found the same species pokemon and it was also shiny. Assuming this is a glitch where once a pokemon is rolled shiny, the species will be rolled shiny subsequent times, but I've also only run into a second shiny after the first, so it might fix itself after the first time.