Now I do have what I consider to be a compelling data point.

P2 was rejected repeatedly in August/September last year for Inks, after consistently following a reasonable velocity and never being denied before (that we can recall).

Since then, she closed cards in December and March to bring her to two open Chase business cards (one Marriott and one Ink).

She applied for a UA Business card using a public link in April and was instantly approved.

I am now convinced that the key is two or fewer open Chase business cards, assuming that you would otherwise be approved on your merits.