Why would a high-rise domestic hot water system repeatedly accumulate large amounts of air?

In a high-rise domestic hot water system with functioning booster pumps, boilers, valve mixers, recirc pump and AAVs, what would cause large volumes of air to repeatedly accumulate at the top of risers (15+ minutes of air during manual bleeding), requiring frequent manual venting both from the top of the main branch and from the bottom valves of individual risers (2-3 times a week) — and where would you look first for the true air collection point that isn’t being effectively vented?

Context: 30-story condo, domestic cold/hot water risers replaced in 2019. 3 booster pumps on main floor to help push cold water in from the city to top of building. Two gas boilers, with electric circulation system and 2AAVs on top of building in mechanical penthouse. One floor below this penthouse is a smaller room that has 3 water storage tanks, recirc pump, mixing valves, 4 AAVs. Maintenance bleeds from this smaller room manually from a valve on the main branch using a hose to a drain. The say this regularly releases sustained air for ~15 minutes. They bleed for over 30 minutes.

Issues recur weekly - ie cold water never getting hot, or lukewarm water never getting hot, worst after overnight low demand, and affect certain branch sides more than others. Trunk bleeding helps temporarily but doesn’t stop recurrence. At this point they are manually bleeding the system 2-3 times a week, varied from the main branch to the bottom of the individual risers.

Any thoughts, insights or additional questions I can answer - thank you in advance!