I'm fairly sure that orb is a portal and the indrik is teleporting out /jumping forward and out from it. The Evergloam Indrik does it too, and I'm pretty sure the normal ones as well... though I think it's more subtle for them (I'll check next time I log in).

You can see this teleporting in a few of the boss fights involving indriks though, the faun boss Glemyos Wildhorn comes to mind.

Not bugged, as rootkitronin said, that's how it's meant to be. I prefer it to mounts growing out of the ground or fading in.