There are two situations where it is now expected to have a confirmed segment in B class which didn't exist when the elite benefit of "instant upgrades on Y/B (and M) fares" was designed:

Delegated Premium Economy fares with a primary rbd of O/A/R on UA generally book into B class on flights without a Premium Economy cabin. The main reason for this is because B class segments grant free Economy Plus access, and United wanted to permit this for such segments. Partner PE fares have historically also delegated to Y, B, or M classes as well, although this predates Economy Plus access being a benefit of B fares and also predates UA having a PE cabin. Note that Business fares almost always delegate to Y class, and do not then receive E+ access on, for example, one-class United flights.

Economy Plus fares, which are published systemwide (or near-to) domestically as a differential fare to regular Economy and a primary rbd of B. These fares have an 'I' in seventh position (which usually identifies the differential type of a domestic fare), and are sold specifically to "bundle" Economy Plus access as part of the coupon value itself. So far, United has not marketed them, but searching for B class on a domestic route should yield this as the cheapest fare which confirms in B class.

UA has also begun inventory controlling B class based on assigned E+ seats.

It is not known if either, both, or neither of the above fare types are intended to receive Instant Upgrade benefits - the general bugginess of the system makes it difficult to gather data.