Drainback shouldn't be an issue since the bypass system can only use as much oil as the orifice will allow. If you have the orifice close to the engine, you won't even have the delay of filling the oil lines before engine oil pressure builds up.
You are wrong in both posts, Joe. What you are saying would perhaps be true if the bypass system involved here was the BMK-21 single filter system, which is ONLY a bypass (low flow) filter.
The system we're dealing with here is the dual filter BMK-25 system, which is a full flow AND bypass filter on the same mount. This system takes normal, full oil pump throughput to and from the unit at all times since this is unit houses a full flow (Eao26) filter. Inside the unit there is a restrictor that limits some of that full throughput into the EaBP100 bypass filtration element. But since the lines to and from the system are large diameter for full flow, and since there is no restrictor in the line, this system can bleed back.
Oil flows into the BMK-25 and it all passes through the full flow Eao26 filter first. The majority of that oil goes straight back to the engine, while a small portion of that oil throughput is routed through the bypass element for finer filtering.
What you are seeing is the full flow filter draining back and the lines emptying. I bet if you spun off both filters with the truck off, the bypass filter would be low or empty and the bypass filter (EaBP100) would be full.