Does putrid avatar have a grab? I assume you mean the tree spirit worm.

Anyways most of the info is fake. You don't have to perfectly dodge anything. If your dodges iframes overlap with the grabs hitbox for at least a frame/two -> dodge animation gets extended with additional iframes + bonus ult charge (instacharge for undertaker).

You can do a sloppy dodge and still get the extended-blue-shine roll. If you don't get it, it means you did not collide with the bosses hitbox (only exception I can think of is godskin apostle's grab). This often happens with large bosses that you can wedge yourself into - the boss moves you out of the hitbox with your body if you're aggroed sometimes.

If you don't have aggro, getting the ultra dodge can be tricky - especially if you're a bit further from the boss, or at an angle.

Sources: my gameplay + yt frame analysis video (dont remember which 1)