The OR braking physics doesn't work well with AI trains. Probably due to the high frequency apply/release cycles, the auxiliary reservoir pressure drops down to the full service pressure and has no more enough time to recharge, while the BC pressure goes to 0. In this situation braking is no more possible, because the brake pipe pressure can't drop below the AR pressure.
Due to this various workarounds have been inserted in the AI train movement logic. One of these workarounds forces speed at 0 when the AI train must stay stopped at stations or before signals, even on a slope.
This workaround isn't present for waiting points, even if linked with a signal. So, if an AI train stops before a red signal because of a WP, it drifts away and passes the signal.
Fixed in x.3203.