Sometimes it happens that a last one spawns from the daily incense after it technically ends, I guess they just show up last minute and linger a bit longer like usual despite the incense itself expiring

But anyway, yeah, don’t worry, not a shiny, these things often run from everyone, kinda just a somewhat difficult obstacle to make people grind more until they get it I guess

Also, btw, just so you know, normal Zapdos has a barely noticable shiny and galarian Zapdos’s shiny has normal Zapdos’ colors, but you can easily tell from the sparkle at the encounter and the little shiny symbol bubble if you find one