I noticed that when the failed milestone is trying to trigger, the infant is thinking of a nearby chair, but since they're not close enough, it fails. When it tries to trigger and the infant is close enough to a chair, it finally works.
I would expect that, rather than outright failing, the infant would scoot or crawl over to the chair and succeed in the milestone, rather than simply failing like it currently does.
Update, some months later: Yes, it's the availability of a chair. I've had several infants get this milestone now. You need to have a rocking chair or living room chair right near the infant or they don't get the pull-to-stand. It might work with a dining chair, but only if it's not attached to a table or desk. I've never seen it work with a dining chair myself.
No chair within a square of the infant? No milestone.
