It actually can flee under special conditions. If Gotcha’s, GoPlus and other catching devices are used and they fail to catch it in one ball, it will instantly run. If the game detects high speed or an accidental GPS drift you’ll also have exactly one try to catch it or else it’ll run as well (hence why I waited about 10 minutes before attempting to catch it since I was in fact in the highway when it spawned). If the game crashes it will not run but it will also be a loss.
Assuming normal conditions and the game working as it should, they can indeed not flee, but they will break out quite a lot.
Also 6 Shiny Horsea, 6 Shiny Trapinch, 2 Shiny Swablu.
Only played last Saturday’s Research Day because I still needed Shiny Skrelp. Came home however with… everything else.
I even had a gut feeling that Tangela was going to be shiny because I always find Shiny Tangelas everywhere, and lo and behold. I wasn’t even surprised.
A very nice exception, when most of the rest of my Legendaries last week have been 
